Search found 60 matches

by Wil Chai 3D
Thu Jan 14, 2021 8:53 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: ICE Tables
Replies: 12
Views: 64

Re: ICE Tables

It all depends on what the initial concentrations are. If the problem gives just initial [reactant] and no product, you know the reactants are going to be subtracted so it would be -x or -2x or whatever matches to stoich coefficient. However, if you are given both initial [reactant] and [product] yo...
by Wil Chai 3D
Wed Jan 13, 2021 12:30 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Ka (Acidity Constant)
Replies: 5
Views: 27

Re: Ka (Acidity Constant)

I think about Ka as the strength of a particular acid. The stronger the acid, the more of it dissociates into its ions; thus we can refer to Ka as the "ionization constant." Hope this makes sense!
by Wil Chai 3D
Tue Jan 12, 2021 9:29 am
Forum: Student Social/Study Group
Topic: Comfort Movies
Replies: 108
Views: 237

Re: Comfort Movies

I love the Jump Street movies with Jonah Hill, Channing Tatum, and Ice Cube. South Park and the Office are great comfort shows too and they're on Comedy Central all the time. The Mission Impossible movies are also amazing.
by Wil Chai 3D
Tue Jan 12, 2021 9:11 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Approximation of X in cubic equations
Replies: 5
Views: 22

Re: Approximation of X in cubic equations

The rule of thumb he mentioned was that we could use the approximation whenever K<10^-4.
by Wil Chai 3D
Tue Jan 12, 2021 9:07 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Neglecting Water
Replies: 20
Views: 84

Re: Neglecting Water

There is so much water in solution in comparison to H3O+ and OH- that the change in concentration of water is appreciable. In other words, most of the water stays as water, so its concentration remains essentially unchanged, which is why you leave it out of the Kw expression.
by Wil Chai 3D
Sat Jan 09, 2021 10:38 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Inert Gas Pressure Change
Replies: 3
Views: 12

Re: Inert Gas Pressure Change

The way I rationalized it was that even if adding inert gas decreases the volume that the other gas particles can occupy, it does this so appreciably that you can say that the volume essentially does not change. Like in Lavelle's example, if you push down the piston halfway, that changes the volume ...
by Wil Chai 3D
Sat Jan 09, 2021 9:42 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Kp vs. Kc
Replies: 3
Views: 16

Re: Kp vs. Kc

Since Kp is evaluated using partial pressures, I'm guessing you could use the ideal gas law to convert those partial pressures to concentrations, and then use those concentrations to calculate Kc.
by Wil Chai 3D
Thu Jan 07, 2021 10:44 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Homework #3
Replies: 3
Views: 25

Re: Homework #3

For this question, it would help to set up an ICE table. I think Prof Lavelle is going to go over this type of problem on Friday. It looks like this: H2(g)+I2(g)↽−−⇀2HI(g Initial. .3 .3 0 Change. -x -x +2x Equilib. .3-x .3-x 2x (Formatting looks off but each number should line up left to right with ...
by Wil Chai 3D
Wed Jan 06, 2021 12:53 pm
Forum: Ideal Gases
Topic: Units of Temperature
Replies: 82
Views: 285

Re: Units of Temperature

if you ever forget, you can just look at the units of the ideal gas constant, which always has K in the denominator.
by Wil Chai 3D
Tue Jan 05, 2021 11:28 am
Forum: Student Social/Study Group
Topic: Advice for someone who didn't take 14A with professor Lavelle
Replies: 61
Views: 253

Re: Advice for someone who didn't take 14A with professor Lavelle

Do textbook problems!! Many of the exam questions are very similar (if not the same) compared to the textbook ones, which Prof Lavelle has said himself. In 14A, I often got exam problems that looked familiar to those found in the textbook.
by Wil Chai 3D
Wed Dec 09, 2020 5:38 pm
Forum: Student Social/Study Group
Topic: Post All Chemistry Jokes Here
Replies: 8503
Views: 1469336

Re: Post All Chemistry Jokes Here

Image
by Wil Chai 3D
Wed Dec 09, 2020 5:29 pm
Forum: Naming
Topic: Aqua vs Hydrate
Replies: 12
Views: 79

Re: Aqua vs Hydrate

Is it aqua if it is directly connected to the coordination compound while it is called hydrate if it is not? Is there anymore examples of like differing names compared to the original atomic name? Yes, if the water is acting as a ligand within the coordination sphere (inside the brackets) it is ref...
by Wil Chai 3D
Wed Dec 09, 2020 4:24 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: CCl4 polarity
Replies: 2
Views: 14

Re: CCl4 polarity

CCl4 is actually nonpolar, I think. Each C-Cl bond is a polar bond, but because the tetrahedral geometry is shaped in a way that each dipole cancels, the molecule is nonpolar overall. Here is a helpful picture: https://qph.fs.quoracdn.net/main-qimg-2f7b301acc4612db37ce8847733a72d7.webp Hope this mak...
by Wil Chai 3D
Wed Dec 09, 2020 3:53 pm
Forum: Bronsted Acids & Bases
Topic: 6A.9c Bronsted acid/base reactions
Replies: 1
Views: 14

Re: 6A.9c Bronsted acid/base reactions

The Bronsted-Lowry definition of acids says that acids/bases must donate/accept a proton or H+ ion. Since neither CH3COOH nor NH3 do this (with respect to each other), it would not fall under the Bronsted-Lowry definition. Hope this makes sense!
by Wil Chai 3D
Mon Dec 07, 2020 4:55 pm
Forum: Identifying Acidic & Basic Salts
Topic: Difference between pH, pKa, and Ka
Replies: 3
Views: 36

Re: Difference between pH, pKa, and Ka

Ka is the dissociation constant, calculated by the equation [A-][H+] / [HA] . "P" is shorthand for "take the negative log of" so pKa = -log([A-][H+] / [HA]). pH is -log[H+]. In lecture, lavelle mentioned that pKa was referring to a weak acid HA, whereas pH was referring to the so...
by Wil Chai 3D
Thu Dec 03, 2020 1:01 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: Isomers
Replies: 2
Views: 30

Re: Isomers

There is no mention of isomers in any of the outlines or on the syllabus, so it shouldn't show up on the final :)
by Wil Chai 3D
Wed Dec 02, 2020 8:11 pm
Forum: Properties & Structures of Inorganic & Organic Acids
Topic: Acid strength and bond length
Replies: 11
Views: 87

Re: Acid strength and bond length

HBr has a longer bond length because the bond between the H and Br is weaker compared to the bond between H and Cl. Thus, HBr it will more readily dissociate, meaning it will contribute more H+ ions to solution, which means HBr is stronger. Hope this makes sense!
by Wil Chai 3D
Wed Dec 02, 2020 8:04 pm
Forum: Lewis Acids & Bases
Topic: Ligands
Replies: 6
Views: 57

Re: Ligands

An important characteristic of a ligand that I think of often is the fact that it's electron rich. Consequently, it'll (always?) have a lone pair. Another way conceptually to think about it is that a ligand is the same thing as a lewis base, because it donates an electron pair.
by Wil Chai 3D
Tue Dec 01, 2020 7:34 pm
Forum: Interionic and Intermolecular Forces (Ion-Ion, Ion-Dipole, Dipole-Dipole, Dipole-Induced Dipole, Dispersion/Induced Dipole-Induced Dipole/London Forces, Hydrogen Bonding)
Topic: Overall Review of Intermolecular Forces
Replies: 4
Views: 37

Re: Overall Review of Intermolecular Forces

More of a specific to chemistry thing, but the strength of IMFs is related to characteristics of substances such as boiling/melting/freezing point, viscosity, heat capacity, and others. Water, for instance, has a high heat capacity due to H-bonds. When heat is added to water, some energy goes in to ...
by Wil Chai 3D
Mon Nov 30, 2020 5:27 pm
Forum: Student Social/Study Group
Topic: Final Exam Study Tips
Replies: 48
Views: 237

Re: Final Exam Study Tips

Like most of the other people on this thread, I think practice problems from the homework/textbook are your best bet. However, something I like to do as well is change up homework problems or make up my own problems! For example, I might find a compound from the notes, draw its lewis structure, and ...
by Wil Chai 3D
Fri Nov 27, 2020 10:40 am
Forum: Student Social/Study Group
Topic: Final Exam Tips
Replies: 24
Views: 139

Re: Final Exam Tips

I feel like I have the most trouble with the conceptual questions. Does anyone have a good way of understanding the qualitative information? For conceptual things, I find that re-writing my notes is a good way to revisit topics and get a better understanding of them. Similar to this, I like writing...
by Wil Chai 3D
Fri Nov 27, 2020 8:47 am
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Schrodinger's Wave Function Equation
Replies: 4
Views: 41

Re: Schrodinger's Wave Function Equation

Lavelle has said in lecture that we won't be doing any actual calculations using Schrodinger's Wave Function in this class, thankfully.
by Wil Chai 3D
Thu Nov 26, 2020 12:22 pm
Forum: Lewis Structures
Topic: sapling q 17
Replies: 2
Views: 35

Re: sapling q 17

One of the structures can have the carbons in a triangle!
by Wil Chai 3D
Thu Nov 26, 2020 11:28 am
Forum: Administrative Questions and Class Announcements
Topic: Chem Community Posts during Thanksgiving break
Replies: 10
Views: 109

Chem Community Posts during Thanksgiving break

We should have 40 posts by Sunday this week (week 8), correct? So the Thanksgiving holiday does not affect the due date or number of posts?
by Wil Chai 3D
Thu Nov 26, 2020 11:24 am
Forum: Determining Molecular Shape (VSEPR)
Topic: sapling #20
Replies: 3
Views: 10

Re: sapling #20

Even though the atoms in a tetrahedral are not directly across from one another, if each atom is the same (for example in AsO4^3-), then it actually IS symmetrical. The way I try to think about this shape is that it's kind of like 2 bent shapes that are across from each other and rotated. If you pic...
by Wil Chai 3D
Sun Nov 22, 2020 12:09 pm
Forum: Student Social/Study Group
Topic: Study Tips for Final Exam
Replies: 57
Views: 405

Re: Study Tips for Final Exam

Just a specific thing when I'm doing textbook problems, I print out the syllabus and box problems I have trouble with. That way when I'm going back over to review, I know exactly what I should focus on. This helps me to stay efficient and avoid burnout!
by Wil Chai 3D
Tue Nov 17, 2020 2:26 pm
Forum: Administrative Questions and Class Announcements
Topic: Chem 14B Lectures
Replies: 15
Views: 140

Chem 14B Lectures

Will Chem 14B lectures for winter quarter will be asynchronous like they are for 14A this quarter? Or will they be synchronous and have recorded attendance?
by Wil Chai 3D
Mon Nov 16, 2020 4:17 pm
Forum: Administrative Questions and Class Announcements
Topic: MT 2 Material
Replies: 4
Views: 42

MT 2 Material

The 14A website says that the material covered on MT2 will span from the end of MT1 (so quantum numbers, e- configs, trends) to the end of outline 3 (bonding). This means shape and VSEPR will not be tested, correct?
by Wil Chai 3D
Mon Nov 16, 2020 4:10 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Textbook 1E.17 part b
Replies: 3
Views: 15

Textbook 1E.17 part b

For Manganese, why would the first electron to be pulled out come from the 4s subshell if the 4s subshell is filled before 3d?

Also, another general question: for elements in period 4, is the 4s subshell always filled before the 3d subshell? (Not counting exceptions like Cr, Cu)
by Wil Chai 3D
Mon Nov 16, 2020 4:00 pm
Forum: Interionic and Intermolecular Forces (Ion-Ion, Ion-Dipole, Dipole-Dipole, Dipole-Induced Dipole, Dispersion/Induced Dipole-Induced Dipole/London Forces, Hydrogen Bonding)
Topic: Attractive Forces in Atoms & Molecules
Replies: 3
Views: 28

Re: Attractive Forces in Atoms & Molecules

A dipole is present when the molecule is polar. You can figure this out by looking at the electronegativity difference between atoms, so for something like HF or OH you can figure out that those bonds are polar based on periodic trends. (Side note: Lavelle said in lecture that a bond is polar whenev...
by Wil Chai 3D
Sat Nov 14, 2020 3:39 pm
Forum: Student Social/Study Group
Topic: Exercising Our Minds and Bodies
Replies: 103
Views: 500

Re: Exercising Our Minds and Bodies

I've found that running is a great way to clear the mind and recharge mentally. I'm usually more productive after a good run!
by Wil Chai 3D
Tue Nov 10, 2020 6:00 pm
Forum: Dipole Moments
Topic: Ionic Character
Replies: 3
Views: 37

Re: Ionic Character

"Higher ionic character" means that there is more unequal sharing of electrons. So the higher the ionic character, the less equally electrons are shared between the atoms in the bond. This can also be interpreted as a greater electronegativity difference or charge difference between the tw...
by Wil Chai 3D
Tue Nov 10, 2020 5:55 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Ionization Energy
Replies: 2
Views: 19

Re: Ionization Energy

You are correct, ionization energy is one of the trends that was experimentally determined!
by Wil Chai 3D
Tue Nov 10, 2020 5:52 pm
Forum: Dipole Moments
Topic: Sapling Question #13
Replies: 5
Views: 54

Re: Sapling Question #13

I believe you also have to account for the N atoms, which can hydrogen bond to H atoms on water. This gives you a total of eight possible H-bonds: one for each of the H's, two for the O, and one for each N. Hope this helps! Edit: forgot to account for extra electron pair on O.
by Wil Chai 3D
Tue Nov 10, 2020 5:48 pm
Forum: Bond Lengths & Energies
Topic: Induced Dipole-Induced Dipole Interaction
Replies: 3
Views: 32

Re: Induced Dipole-Induced Dipole Interaction

Electrons are constantly moving around the nucleus, and since they're smaller than the nucleus, they can fluctuate. I think of an induced dipole as a temporary dipole that happens when e- density from one molecule influences another molecule. I think Lavelle used N2 as an example, saying that e- den...
by Wil Chai 3D
Sun Nov 08, 2020 12:05 pm
Forum: Formal Charge and Oxidation Numbers
Topic: Oxidation number?
Replies: 8
Views: 77

Re: Oxidation number?

I don't believe we've covered this in lecture as of now (end of week 5), but I remember him mentioning 'oxidation state' a few times when referring to charge.
by Wil Chai 3D
Sun Nov 08, 2020 11:56 am
Forum: Resonance Structures
Topic: Question about Resonance Def
Replies: 5
Views: 34

Re: Question about Resonance Def

The first definition Lavelle gave had to do with lewis structures: "some lewis structures have multiple bonds in different equivalent locations," and that resonance is needed "due to limitations in lewis Structures." Here I think he's basically saying that when you can draw a lew...
by Wil Chai 3D
Fri Nov 06, 2020 4:22 pm
Forum: Octet Exceptions
Topic: Phosphorus and d-Orbital
Replies: 7
Views: 66

Re: Phosphorus and d-Orbital

Regarding the first part of your question, I think about the quantum numbers part of it like this: since valence electrons of Phosphorus belong to the principle energy level n=3, the l quantum number could be 2,1, or 0. This means that those electrons could occupy a d-orbital, because 2 is a possibl...
by Wil Chai 3D
Thu Nov 05, 2020 5:05 pm
Forum: Ionic & Covalent Bonds
Topic: Bond Character
Replies: 6
Views: 39

Re: Bond Character

This is a slightly different interpretation, but when Lavelle mentioned bond character in his lecture on resonance it seemed to me that he was referring to whether the bond was a single bond or double bond. For example, the bonds in NO3- have characteristics of both a single and double bond. Going w...
by Wil Chai 3D
Wed Nov 04, 2020 6:56 pm
Forum: Resonance Structures
Topic: Resonance structures
Replies: 3
Views: 47

Resonance structures

So for clarification, whenever you identify resonance in a lewis structure (by realizing there is more than one location where you can place a specific bond) does that ALWAYS mean that the bond length is an average and that electron density is delocalized? Basically, is resonance synonymous with del...
by Wil Chai 3D
Sun Nov 01, 2020 6:47 pm
Forum: Student Social/Study Group
Topic: Sappling #26
Replies: 3
Views: 49

Re: Sappling #26

Use the mass of the electron, 9.109E-31 kg, for mass, which is given on the formula sheet. Also, make sure you remember to convert Angstroms to meters. Hope this helps!
by Wil Chai 3D
Sun Nov 01, 2020 1:18 pm
Forum: Student Social/Study Group
Topic: Favorite TV shows
Replies: 176
Views: 848

Re: Favorite TV shows

If you haven't watched it already, Avatar: The Last Airbender!! (And Legend of Korra)
by Wil Chai 3D
Wed Oct 28, 2020 5:19 pm
Forum: SI Units, Unit Conversions
Topic: SI Unit for Mass
Replies: 10
Views: 103

Re: SI Unit for Mass

The equations that involve joules as a unit require you to use kg, because 1 J = 1 kg m^2 s^-2 . It's important to note also that all equations with Planck's constant require kg, since h = 6.626x10^-34 J*s.
by Wil Chai 3D
Wed Oct 28, 2020 5:14 pm
Forum: Photoelectric Effect
Topic: Sampling #4
Replies: 3
Views: 39

Re: Sampling #4

If Φ is the amount of joules required to remove 1 electron, just take the total energy of the photons that was given and divide it by the energy required to remove 1 electron (Φ), since one photon removes one electron. Hope this helps!
by Wil Chai 3D
Mon Oct 26, 2020 3:08 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Isoelectronic Atoms and Ions
Replies: 2
Views: 38

Re: Isoelectronic Atoms and Ions

Regarding his example with with F- and He, I think it's because they both have different nuclear charges. Since He has the same number of protons as electrons, it is neutral. But since F- has one more electron than proton, it has a negative charge and could interact electrostatic-ly with positively ...
by Wil Chai 3D
Fri Oct 23, 2020 5:27 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Orbitals
Replies: 9
Views: 100

Re: Orbitals

I think he was talking about the lone unpaired electron in a silver atom, which is in the 5s state. 5s refers to an electron with a principle energy level of n=5 that occupies an s-orbital.
by Wil Chai 3D
Fri Oct 23, 2020 3:10 pm
Forum: Molarity, Solutions, Dilutions
Topic: Fundamental G25
Replies: 2
Views: 46

Re: Fundamental G25

If you double the volume, you half the molarity, and since you do that 90 times you multiply the initial molarity by (1/2)^90. You end up with something so small the textbook just calls it 0, so a solution that dilute wouldn't be very effective.
by Wil Chai 3D
Mon Oct 19, 2020 3:12 pm
Forum: Properties of Light
Topic: Sapling HW Week 2, 3, 4 Question #7
Replies: 10
Views: 118

Re: Sapling HW Week 2, 3, 4 Question #7

This helped a lot, but I am just wondering where you got the equation you used to find the energy of one photon. Is that something you were able to derive on your own or is that a given number that always stays constant for the energy of one photon? He used the equation E=h(c/lamda) which is the sa...
by Wil Chai 3D
Mon Oct 19, 2020 3:03 pm
Forum: General Science Questions
Topic: Midterm Structure
Replies: 4
Views: 65

Re: Midterm Structure

I remember my TA saying it would be all multiple choice.
by Wil Chai 3D
Mon Oct 19, 2020 3:01 pm
Forum: DeBroglie Equation
Topic: Momentum of Photon
Replies: 1
Views: 38

Momentum of Photon

After watching the lecture, I'm confused about how a photon can have momentum if a photon has no mass but p=mv. Dr. Lavelle explained it by saying that in photosynthesis, light transfers energy to electrons in molecules in leaves so it must have a momentum, which kind of makes sense. But then how wo...
by Wil Chai 3D
Wed Oct 14, 2020 2:07 pm
Forum: Einstein Equation
Topic: Wave Model Question
Replies: 5
Views: 71

Re: Wave Model Question

It could help to think about it in terms of the equation E(light/photon) = hv. In order to eject an electron, E(light/photon) must be high enough to meet the threshold Energy. E = hv tells us that the energy of the light is dependent on the frequency v of the photon, not the amplitude.
by Wil Chai 3D
Wed Oct 14, 2020 2:01 pm
Forum: Photoelectric Effect
Topic: 10/14 Lecture Threshold Energy
Replies: 8
Views: 95

Re: 10/14 Lecture Threshold Energy

hi! Threshold energy is basically the energy of the photon being sufficient enough to overcome the binding energy of an e- and atom. The energy of this photon is dependent on the wavelength. I would use the equation: E (photon) = hV OR hc/lambda (wavelength) Hope this helps! Since the equation is E...
by Wil Chai 3D
Wed Oct 14, 2020 1:35 pm
Forum: Properties of Light
Topic: Lecture Music [ENDORSED]
Replies: 6
Views: 122

Re: Lecture Music [ENDORSED]

Chem_Mod wrote::-)
Thanks for the compliment!

I enjoyed it too but you have to bring back Portishead in the future!
by Wil Chai 3D
Mon Oct 12, 2020 3:31 pm
Forum: Photoelectric Effect
Topic: Photoelectric Effect
Replies: 5
Views: 61

Re: Photoelectric Effect

Regarding the equation, you could also think about it in terms of the Law of Conservation of Energy, ie, the total energy of the incoming photon (E photon) = the energy that is used to remove the electron from the metal (E energy to remove e-) plus the energy that is left over (E excess), which is t...
by Wil Chai 3D
Mon Oct 12, 2020 11:47 am
Forum: Properties of Light
Topic: c=λv
Replies: 5
Views: 79

Re: c=λv

Building off some other people's responses: Try putting the equation x=c/y where c is any constant into a a graphing calculator (https://www.desmos.com/calculator). You will see a hyperbola, not an oscillating function. c = lamda*v just tells you that frequency and wavelength are inversely related. ...
by Wil Chai 3D
Fri Oct 09, 2020 4:30 pm
Forum: Empirical & Molecular Formulas
Topic: M.20
Replies: 2
Views: 36

Re: M.20

So the equation I wrote to start of with was: Nicotine + O2 --> CO2 + H2O + N2 Since you know all of the C that was part of nicotine is in now CO2, and similarly you know all the H and N that started in Nicotine is now H2O and N2 respectively, you can use the products to calculate the amount of C H ...
by Wil Chai 3D
Wed Oct 07, 2020 1:29 pm
Forum: Empirical & Molecular Formulas
Topic: Limiting Reactant Post-Module Q.18
Replies: 3
Views: 67

Re: Limiting Reactant Post-Module Q.18

I did it this way: 21.4 g NH3 x (mol NH3 / 17 g) x (5 mol O2 / 4 mol NH3) = 1.57 mol O2 needed to completely react with all the NH3 42.5 g O2 x (mol O2 / 32 g) = 1.33 mol of O2 you actually have Since you would need 1.57 mol O2 to completely react with all the NH3 but you only have 1.33 mol, O2 is l...
by Wil Chai 3D
Mon Oct 05, 2020 2:32 pm
Forum: Limiting Reactant Calculations
Topic: Fundamentals M5
Replies: 7
Views: 82

Re: Fundamentals M5

You can use either reactant, but I used ClO2. 12 mol ClO2 x (2 mol BrF3) / 6 mol ClO2 = 4 mol BrF3, meaning you would need 4 mol of BrF3 to completely use up all 12 mol of ClO2. Since you actually have 5 mol of BrF3, it means this actually happens and all 12 mols of ClO2 will be used up. So the reac...
by Wil Chai 3D
Mon Oct 05, 2020 2:17 pm
Forum: Molarity, Solutions, Dilutions
Topic: Molar mass for O2 vs O
Replies: 11
Views: 116

Re: Molar mass for O2 vs O

The molar mass of a single O atom is 16 g/mol, so to calculate the molar mass of O2 which is 2 O's you do 16 + 16.
by Wil Chai 3D
Mon Oct 05, 2020 1:45 pm
Forum: Empirical & Molecular Formulas
Topic: Fundamentals Problem E15
Replies: 4
Views: 44

Re: Fundamentals Problem E15

The OH- ion is has a charge of -1, and since there are 2 of them in the molecule you know metal 'M' must have a charge of +2. Looking at the periodic table you can see that the sulfide ion would have a charge of -2, so the formula of this metal's sulfide would be MS. Now just add up the molar masses...

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