## Search found 67 matches

Tue Jan 12, 2021 6:22 am
Forum: Administrative Questions and Class Announcements
Topic: Participation
Replies: 18
Views: 84

### Re: Participation

Here's an explanation of how chem community posts work! https://lavelle.chem.ucla.edu/wp-conten ... ussion.pdf
Tue Jan 12, 2021 6:12 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: K in relation to shifting right/left
Replies: 8
Views: 33

### Re: K in relation to shifting right/left

Reactions with a very small K value (K < 10^-3) lies to the left because the reactant concentration is larger than the product concentration at equilibrium. Reactions with a very large K value (K > 10^3) lies to the right because the product concentration is larger than the reactant concentration at...
Tue Jan 12, 2021 6:02 am
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Getting two positive x values when using quadratic
Replies: 41
Views: 138

### Re: Getting two positive x values when using quadratic

If you get two positive values test them both by plugging them into the equilibrium concentration expression you got from your ICE table. Usually, one should give you a negative value which is not possible, and therefore the other one would be the x value.
Tue Jan 12, 2021 5:57 am
Forum: General Science Questions
Topic: Acid & Base Equilibria Lecture Question
Replies: 7
Views: 51

### Re: Acid & Base Equilibria Lecture Question

H2O is not included because it is in large excess which means that its concentration remains almost the same throughout the reaction. Therefore, it is not included in the Kw expression.
Tue Jan 12, 2021 5:53 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Kc vs Kp
Replies: 6
Views: 30

### Re: Kc vs Kp

I believe you can as long as you make sure that the reactants and products in the equilibrium constant equation are either all partial pressures or all concentrations.
Wed Jan 06, 2021 5:18 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: molar concentrations
Replies: 5
Views: 18

### Re: molar concentrations

If you are given only moles and volume, you would find the concentration through moles/volume. If you are given partial pressures, you would find the concentration by PV=nRT --> n/V=P/RT.
Wed Jan 06, 2021 5:09 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Exothermic vs Endothermic Reactions
Replies: 6
Views: 34

### Re: Exothermic vs Endothermic Reactions

Yes, in exothermic reactions energy is released so the delta H is negative, while in endothermic reactions energy is absorbed so the delta H is positive.
Wed Jan 06, 2021 5:03 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: solids and liquids in k expression
Replies: 4
Views: 38

### Re: solids and liquids in k expression

Pure solids have no concentration (molL-1) because they are not dissolved in any solvent. Pure liquids are not included because their effective concentrations do not change in a reaction.
Wed Jan 06, 2021 4:53 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Q compared to K
Replies: 6
Views: 20

### Re: Q compared to K

I believe as long as you are looking at the same reaction with the same conditions, K does not change. So when comparing Q to K there are only three possibilities: Q equals K, is larger than K, or smaller than K.
Wed Jan 06, 2021 4:49 pm
Forum: Ideal Gases
Topic: K and PV=nRT
Replies: 9
Views: 57

### Re: K and PV=nRT

I think it would depend on the problem and you would use the necessary conversions to get the answer you need. For instance, if you are asked to find the partial pressure of the product given its equilibrium concentration, you would use PV=nRT to convert concentration to pressure.
Wed Dec 09, 2020 6:22 am
Forum: Determining Molecular Shape (VSEPR)
Topic: Bond Angles
Replies: 5
Views: 54

### Re: Bond Angles

The angles depend on whether the atoms are all in the same place or not. For instance, in the tetrahedral geometry the atoms are not in the same place so the angles would not add up to 360º. While in the trigonal planar geometry they are so the bond angles would add up to 360º.
Wed Dec 09, 2020 6:18 am
Forum: Bronsted Acids & Bases
Topic: Amphoteric compounds
Replies: 8
Views: 68

### Re: Amphoteric compounds

I agree with the above reply. I would first look to see if the compound contains protons and if it does it is an acid. Then try to see if once you donate the proton if the conjugate base can accept a proton.
Wed Dec 09, 2020 6:12 am
Forum: Naming
Topic: [Co(NH3)5 Cl] Cl 2H2O
Replies: 5
Views: 48

### Re: [Co(NH3)5 Cl] Cl 2H2O

I believe it is because it is out of the coordination sphere. Just like how chlorine is named chloro if it is inside the coordination sphere and chloride if it is outside the sphere.
Wed Dec 09, 2020 6:10 am
Forum: Lewis Acids & Bases
Topic: Lecture 28 #1
Replies: 5
Views: 24

### Re: Lecture 28 #1

I think the example is showing how the fluorine atom in the acid (not HF) is stabilizing the anion because it has high electron withdrawing ability.
Wed Dec 09, 2020 6:05 am
Forum: Bronsted Acids & Bases
Topic: HF
Replies: 15
Views: 98

### Re: HF

You would classify a compound being a strong acid if it gives up its proton readily. F is very electronegative so it holds onto its H atom strongly and the bond cannot break easily which means that it cannot donate its proton easily. Therefore HF is a weak acid.
Wed Dec 09, 2020 5:59 am
Forum: Shape, Structure, Coordination Number, Ligands
Topic: Square Planar vs. Tetrahedral
Replies: 15
Views: 123

### Re: Square Planar vs. Tetrahedral

I believe you don't have to know which one when looking at coordination compounds with a coordination number of 4, but you may want to remember some of the biological examples like the heme complex that is a square planar.
Mon Nov 30, 2020 11:15 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Bond Angles
Replies: 13
Views: 92

### Re: Bond Angles

I would look at the number of lone pairs. If lone pairs are present it means they repel the bonding pairs making the bond angles smaller than without the presence of lone pairs.
Mon Nov 30, 2020 11:03 pm
Forum: Hybridization
Topic: identification
Replies: 5
Views: 66

### Re: identification

I agree with the above reply. I would first look at the number of regions of electron density around the central atom. Then I would know how many hybrid orbitals are needed.
Mon Nov 30, 2020 11:01 pm
Forum: Hybridization
Topic: pcl3
Replies: 8
Views: 48

### Re: pcl3

Regions of electron density = # of hybrid orbitals. For PCl3 there are 4 regions of electron density around the P central atom, therefore you would need 4 hybrid orbitals, so the P would have sp3 hybridization.
Mon Nov 30, 2020 10:52 pm
Forum: Naming
Topic: Sapling Q1
Replies: 20
Views: 178

### Re: Sapling Q1

I believe the cobalt has an oxidation state of +3 and not +2 because each Cl has a -1 charge and NH3 is a neutral compound.
Mon Nov 30, 2020 10:46 pm
Forum: Naming
Topic: Tips for Naming Coordination Compounds
Replies: 6
Views: 36

### Re: Tips for Naming Coordination Compounds

Dr. Lavelle had this document on his website that may be useful! https://lavelle.chem.ucla.edu/wp-conten ... pounds.pdf
Wed Nov 25, 2020 6:45 am
Forum: Determining Molecular Shape (VSEPR)
Topic: Reason why lone pairs decrease bond angle
Replies: 7
Views: 45

### Re: Reason why lone pairs decrease bond angle

I usually imagine it as the lone pairs being really repulsive that it pushes the atoms closer together causing the bond angles between them to decrease.
Wed Nov 25, 2020 6:43 am
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Determining an orbital
Replies: 10
Views: 87

### Re: Determining an orbital

I agree with the above post. Just remember that l=0 is an s orbital, l=1 is a p orbital, l=2 is a d orbital, and l=3 is a f orbital.
Wed Nov 25, 2020 6:39 am
Forum: Determining Molecular Shape (VSEPR)
Topic: axial vs. equatorial
Replies: 5
Views: 65

### Re: axial vs. equatorial

When lone pairs occupy equatorial positions, it only repels two axial bonds at 90º, whereas if the lone pair occupied the axial position it would interact with 3 equational bond's. Therefore, lowest energy is achieved when lone pairs are in equatorial position because there is less repulsion.
Wed Nov 25, 2020 6:32 am
Forum: Determining Molecular Shape (VSEPR)
Topic: Textbook 2E #17
Replies: 3
Views: 16

### Re: Textbook 2E #17

Ozone has 3 electron density regions so the electron geometry would be a trigonal planar while the molecular geometry would be bent. Therefore, the bond angle would be less than 120º.
Wed Nov 25, 2020 6:27 am
Forum: Hybridization
Topic: Finding Hybridization
Replies: 6
Views: 26

### Re: Finding Hybridization

Not sure if there are, but if you look at the outline 4 on Dr. Lavelle's website it says which types of hybridization you should know.
Sat Nov 21, 2020 3:32 am
Forum: Formal Charge and Oxidation Numbers
Topic: Best Formal Charge Equations
Replies: 24
Views: 129

### Re: Best Formal Charge Equations

I usually calculate formal charge by FC = valence electrons - number of dots - number of bonds/lines.
Sat Nov 21, 2020 3:30 am
Forum: Lewis Structures
Topic: Identifying Lewis Acids and Bases
Replies: 8
Views: 57

### Re: Identifying Lewis Acids and Bases

I would draw the lewis structures of the molecules involved in the equation. Then from there, generally the species with lone pairs of electrons would be the lewis base and the other would be the lewis acid.
Sat Nov 21, 2020 3:28 am
Forum: Dipole Moments
Topic: Polarity
Replies: 30
Views: 152

### Re: Polarity

If dipole moments occurring between atoms in a molecule cancel out, then you would say there is no net dipole moment making the molecule non polar.
Sat Nov 21, 2020 3:25 am
Forum: Polarisability of Anions, The Polarizing Power of Cations
Topic: Determining Polarity
Replies: 6
Views: 51

### Re: Determining Polarity

I think of polarity as simply a difference in electronegativity between two atoms because the difference will result in one atom pulling the electron density towards themselves leading to a dipole moment. To determine whether a molecule is polar you would look at whether the dipole moments cancel ou...
Sat Nov 21, 2020 3:18 am
Forum: Determining Molecular Shape (VSEPR)
Topic: Polar or Nonpolar
Replies: 13
Views: 220

### Re: Polar or Nonpolar

After determining the shape, a molecule will be polar if the dipole moments are not equal and are not opposite of each other.
Thu Nov 12, 2020 7:10 am
Forum: Bond Lengths & Energies
Topic: 2D #13
Replies: 2
Views: 29

### Re: 2D #13

I did the same and first drew the the lewis structures, then compared the bond lengths of the CO, SO, and CN bond in each molecule.
Thu Nov 12, 2020 7:03 am
Forum: General Science Questions
Topic: Which would have lower ionization energy?
Replies: 13
Views: 118

### Re: Which would have lower ionization energy?

Chlorine would have a lower ionization energy compared to oxygen because ionization energy decreases down the periodic table.
Thu Nov 12, 2020 6:52 am
Forum: Electron Configurations for Multi-Electron Atoms
Topic: 2A.9 Which M2+ are predicted to have following ground state e- configuration
Replies: 1
Views: 8

### Re: 2A.9 Which M2+ are predicted to have following ground state e- configuration

The 2+ indicates that 2 electrons are lost. As the outermost electrons are lost, in question a) it means that the 2 electrons in the 4s orbital is lost, which means that you need to find the element with 7 electrons in the 3d orbital which is Co.
Thu Nov 12, 2020 6:46 am
Forum: Dipole Moments
Topic: induced dipole vs dipole
Replies: 11
Views: 42

### Re: induced dipole vs dipole

I believe dipole-dipole is the interactions between polar molecules, and induced dipole-induced dipole are always present in all interactions between molecules.
Thu Nov 12, 2020 6:41 am
Forum: Lewis Structures
Topic: Sapling hw 3
Replies: 11
Views: 83

### Re: Sapling hw 3

Nitrogen cannot have an expanded octet as it does not have d orbitals that can accommodate the additional electrons. Which means that the nitrogen needs to follow the octet guideline, so there is only one N=O bond.
Wed Nov 04, 2020 7:53 am
Forum: Trends in The Periodic Table
Replies: 8
Views: 60

In general, if the ions are isoelectronic it means they have the same number of electrons, so their ionic radius will depend on the nuclear charge which is affected by the number of protons. The greater the number of protons, the more attracted the electrons are to the nucleus, thus the smaller the ...
Wed Nov 04, 2020 7:47 am
Forum: Electron Configurations for Multi-Electron Atoms
Topic: paired and parallel
Replies: 10
Views: 55

### Re: paired and parallel

When two electrons are paired they have opposite spins. When two electrons are parallel they have the same spin.
Tue Nov 03, 2020 8:14 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Sapling 1E.23
Replies: 3
Views: 25

### Re: Sapling 1E.23

You write electron configurations in the order of increasing energy. When the 3d orbital is unoccupied it has higher energy than 4s, so you would fill in the 4s first. But once the 3d orbital is occupied, 4s has higher energy, so when writing electron configurations you write 4s after 3d.
Tue Nov 03, 2020 8:11 pm
Forum: Formal Charge and Oxidation Numbers
Topic: Formal Charge and Resonance Structures
Replies: 4
Views: 44

### Re: Formal Charge and Resonance Structures

In general you would want to have as many atoms with a formal charge of 0 in a molecule. So, it would be better for the oxygen to have a formal charge equal to -1.
Tue Nov 03, 2020 8:09 pm
Forum: Trends in The Periodic Table
Topic: Why is the ionization energy of O lower than N?
Replies: 13
Views: 90

### Re: Why is the ionization energy of O lower than N?

Oxygen has a lower ionization energy than nitrogen because oxygen has two paired electrons in one of its 2p subshell, while nitrogen does not. Therefore, the two paired electrons in oxygen experience electron-electron repulsion making it easier to remove an electron compared to the nitrogen which ha...
Tue Nov 03, 2020 8:06 pm
Forum: Lewis Structures
Topic: Lewis Structure
Replies: 7
Views: 49

### Re: Lewis Structure

I believe you would put the element with the lowest ionization energy in the middle, and then arrange the other elements around that central element.
Sat Oct 31, 2020 3:42 am
Forum: Quantum Numbers and The H-Atom
Topic: Spin state
Replies: 25
Views: 172

### Re: Spin state

I believe the spin state of an electron tells us whether the electron is upwards or downwards. And this relates to the Pauli exclusion principle, where the electrons in an orbital must have opposite spins.
Sat Oct 31, 2020 3:36 am
Forum: Trends in The Periodic Table
Topic: Ionization energy
Replies: 4
Views: 57

### Re: Ionization energy

Oxygen has a lower ionization energy than nitrogen because in oxygen there is electron electron repulsion in one of the p orbitals making it more easy to remove an electron. Fluorine has a greater ionization energy than oxygen because fluorine has a greater effective nuclear charge making it more di...
Sat Oct 31, 2020 3:24 am
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Shape of Orbitals
Replies: 8
Views: 67

### Re: Shape of Orbitals

I believe the shapes are from the wave functions that work from the Schrodinger equation. Therefore since there are many wave functions that work, there are different shapes.
Sat Oct 31, 2020 3:13 am
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Electron Configuration Shorthand Question
Replies: 5
Views: 44

### Re: Electron Configuration Shorthand Question

I think there's no rule against using the shorthand notation, but if the question is asking for the 'full' electron configuration I believe you would not use the shorthand notation.
Sat Oct 31, 2020 3:09 am
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Clarification on ml quantum number
Replies: 7
Views: 71

### Re: Clarification on ml quantum number

ml tells you the number and orientation of orbitals in a subshell with the possible values being ml=-l...+l. The number of ml values you get corresponds to how many orbitals of that type exists. For instance, if l=1 (p orbital) and ml=-1,0,1, you have 3 ml values which means you have 3 p orbitals px...
Sat Oct 31, 2020 3:06 am
Forum: Quantum Numbers and The H-Atom
Topic: Sapling #28 Homework
Replies: 3
Views: 48

### Re: Sapling #28 Homework

I write out the possible values by ml=-l...l. But also remember that the number of s orbitals is one, p orbitals is 3, d orbitals is 5 and so forth increasing by 2 to make sure my answer is correct in the end.
Wed Oct 21, 2020 3:12 am
Forum: DeBroglie Equation
Topic: Use
Replies: 4
Views: 143

### Re: Use

I think something to look out for when wondering if you should use the De Broglie equation is looking to see if the question talks about masses and frequencies as those are in the equation.
Wed Oct 21, 2020 3:07 am
Forum: Properties of Electrons
Topic: Post Assesment Module: Wave Properties of Electrons and the De Broglie Equation Number 35
Replies: 6
Views: 76

### Re: Post Assesment Module: Wave Properties of Electrons and the De Broglie Equation Number 35

At a UA session they said: detectable wavelike properties>10^-15m>not detectable. And as 6.94 x 10-38 m is smaller than 10^-15m the answer is no.
Wed Oct 21, 2020 2:58 am
Forum: Properties of Light
Topic: Longer wavelength means larger wavelength?
Replies: 5
Views: 52

### Re: Longer wavelength means larger wavelength?

I agree with the others. I believe larger wavelength is referring to longer wavelength. Also, I always try to understand the context of the question to make sure everything makes sense when I get confused on these terminology stuff.
Wed Oct 21, 2020 2:55 am
Forum: Quantum Numbers and The H-Atom
Topic: Lymer and Balmer series
Replies: 5
Views: 49

### Re: Lymer and Balmer series

Jamie Wang 3G wrote:How will you know if the number given by the series are the initial or final level? is it just based on the context of the question?

I think it would depend on whether the question is taking about the emission spectra or absorption spectra.
Wed Oct 21, 2020 2:52 am
Forum: DeBroglie Equation
Topic: Measureable-wavelike properties
Replies: 5
Views: 66

### Re: Measureable-wavelike properties

I believe its: detectable wavelike properties>10^-15m>not detectable
Thu Oct 15, 2020 8:38 pm
Forum: Photoelectric Effect
Topic: 1B #7 part b
Replies: 5
Views: 53

### Re: 1B #7 part b

Part (a) gives you the energy for 1 sodium atom. In part (b) you are asked to find the energy emitted by 5.00mg of sodium atoms. So you first have to find the number of sodium atoms there are in 5.00mg by using Avogadro's constant. Then you can multiply the number of sodium atoms in 5.00mg you found...
Thu Oct 15, 2020 7:18 am
Forum: Einstein Equation
Topic: m vs nm
Replies: 66
Views: 433

### Re: m vs nm

The formula you use in the calculations can also give you an idea of which to use. For instance, in the equation c=lambda*v, the units for the speed of light is meters per second so you would want the unit of lambda to be meters as the units of v is s^-1.
Wed Oct 14, 2020 6:32 pm
Forum: Properties of Light
Topic: Example 1 in Wk 2 Lec 2
Replies: 1
Views: 21

### Re: Example 1 in Wk 2 Lec 2

E=hv
c= lambda*v --> v=c/lambda

Substitute c/lambda for v in the E=hv equation:
E=hc/lambda

Then rearrange to have it equal to lambda:
lambda=hc/E
Wed Oct 14, 2020 8:23 am
Forum: Properties of Light
Topic: Photoelectric spectrum
Replies: 24
Views: 163

### Re: Photoelectric spectrum

I agree, I think its a good idea to remember the wavelengths for visible light and have a general idea of the order of the spectrum.
Wed Oct 14, 2020 8:13 am
Forum: Photoelectric Effect
Topic: Photoelectric Module #29 Sig. Figs.
Replies: 2
Views: 48

### Re: Photoelectric Module #29 Sig. Figs.

I agree, I think its 3 sig figs too.
Wed Oct 14, 2020 8:12 am
Forum: Properties of Light
Topic: Textbook Homework 1A.9
Replies: 4
Views: 33

### Re: Textbook Homework 1A.9

1MHz (megahertz) = 10^6 Hz
Sun Oct 11, 2020 5:38 am
Forum: Significant Figures
Topic: When should I start looking at sig figs?
Replies: 31
Views: 231

### Re: When should I start looking at sig figs?

I don't change the sig figs in any of the values used in the calculation until I get to the answer. Once I find the final answer, I make sure the sig figs of the answer matches the lowest sig figs among the measured values.
Thu Oct 08, 2020 3:30 pm
Forum: Molarity, Solutions, Dilutions
Topic: Fundamentals L: #37
Replies: 5
Views: 65

### Re: Fundamentals L: #37

(b) This is an acid-base neutralization reaction. If we write the balanced equation for the neutralization of NaOH with HNO3, NaOH and HNO3 has a 1:1 ratio so we could use the same equation as (a). V*0.20M=0.1L*0.50M V=0.25L=2.5*10^2mL Hi! I was wondering how you knew that this was an acid-base neu...
Thu Oct 08, 2020 3:25 pm
Forum: Molarity, Solutions, Dilutions
Topic: A solution of CaCl2 in water forms a mixture that is 41.5% calcium chloride by mass. If the total mass of the mixtur
Replies: 4
Views: 35

### Re: A solution of CaCl2 in water forms a mixture that is 41.5% calcium chloride by mass. If the total mass of the mi

I also did the same. I first multiplied the percentage of calcium chloride with the total mass of the mixture which gave the mass of calcium chloride. Then I subtracted the mass of calcium chloride found from the mass of the total mixture which gave the mass of water.
Thu Oct 08, 2020 1:20 am
Forum: Bond Lengths & Energies
Topic: Bond Length
Replies: 22
Views: 348

### Re: Bond Length

Yes. Because triple bonds are the strongest, they require more energy compared to single and double bonds to break.
Thu Oct 08, 2020 1:10 am
Forum: Molarity, Solutions, Dilutions
Topic: Fundamentals L: #37
Replies: 5
Views: 65

### Re: Fundamentals L: #37

(b) This is an acid-base neutralization reaction. If we write the balanced equation for the neutralization of NaOH with HNO3, NaOH and HNO3 has a 1:1 ratio so we could use the same equation as (a).
V*0.20M=0.1L*0.50M
V=0.25L=2.5*10^2mL
Mon Oct 05, 2020 9:38 pm
Forum: General Science Questions
Topic: Meaning of a notation
Replies: 4
Views: 44

### Re: Meaning of a notation

It means directly proportional.
Mon Oct 05, 2020 8:56 pm
Forum: Significant Figures
Topic: Sig Figs
Replies: 11
Views: 118

### Re: Sig Figs

Here are the sources Dr. Lavelle provided on his website about sig figs.
https://lavelle.chem.ucla.edu/wp-conten ... OUT_SF.pdf
https://lavelle.chem.ucla.edu/wp-conten ... ussion.pdf
Mon Oct 05, 2020 6:58 am
Forum: Significant Figures
Topic: Fundamentals G
Replies: 6
Views: 168

### Re: Fundamentals G

As the volume is doubled, the concentration is halved, therefore the final concentration after 90 doublings is: 0.10M*(1/2^90)=8.08*10^-29M. The number of moles in 10ml of the final solution is: 8.08*10^-29M*0.010L=8.08*10^-31 mol. Then you can calculate the number of molecules present in 10ml of th...