## Search found 70 matches

Wed Mar 13, 2019 4:34 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Test 2 #6 Ordering Reducing/Oxidizing Power
Replies: 3
Views: 215

### Re: Test 2 #6 Ordering Reducing/Oxidizing Power

Strongest reducing power: most negative value Strongest oxidation power: most positive value For C, find the most negative value (reducing power) as your anode because you'll be subtracting it in the formula [E°(cell) = E°(cathode) − E°(anode)] and the most positive value (oxidation power) as your c...
Wed Mar 13, 2019 4:20 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Test 2 Cell Diagram
Replies: 3
Views: 180

### Re: Test 2 Cell Diagram

Also, if there is a solid in the reaction, make sure it is conducting (metal). If not, we then add Pt(s).
Wed Mar 13, 2019 4:18 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Platinum in Cell Diagram
Replies: 7
Views: 337

### Re: Platinum in Cell Diagram

We add Pt(s) if the reaction has no conducting (metal) solid.
Sun Mar 10, 2019 12:58 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Test 2: Melting Ice
Replies: 6
Views: 432

### Re: Test 2: Melting Ice

Melting of ice is a spontaneous reaction, it releases heat. Therefore, it has a negative deltaG.
You can also think about it another way. Liquid water has more entropy than solid ice. Thus, it can imply that deltaG is more (-).
Sun Mar 10, 2019 12:49 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Water in a cell diagram
Replies: 5
Views: 191

### Re: Water in a cell diagram

My TA said that you don't include water in the cell diagram because with a galvanic cell, it is implied there's already water.
Sun Mar 10, 2019 12:43 pm
Forum: Balancing Redox Reactions
Topic: Reducing power?
Replies: 3
Views: 221

### Re: Reducing power?

Just remember that:

Strongest reducing power: most negative value
Strongest oxidation power: most positive value
Sun Mar 03, 2019 8:52 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Pt(s)
Replies: 10
Views: 330

### Re: Pt(s)

Also, even if the equation gives you a solid, make sure it's a metal. If not, then you have to use Pt(s).
Sun Mar 03, 2019 8:48 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Cell diagrams and solids
Replies: 9
Views: 320

### Re: Cell diagrams and solids

You would want a metal solid to be the electrode. Pt(s) is inert so it's the usual go-to.
Sun Mar 03, 2019 8:43 pm
Forum: Balancing Redox Reactions
Topic: Strength of reducing agent
Replies: 10
Views: 398

### Re: Strength of reducing agent

Also, remember that the reducing agent is associated with the oxidation reaction.
Sat Feb 23, 2019 10:39 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Galvanic Cell Diagram
Replies: 3
Views: 122

### Re: Galvanic Cell Diagram

Also, the cell diagram itself might be given and we're asked to provide the half-reactions, similarly to problem 14.11 in the 6th Ed. So, you should also know how to work backwards regarding cell diagrams.
Sat Feb 23, 2019 10:36 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Galvanic Cell Diagram
Replies: 6
Views: 187

### Re: Galvanic Cell Diagram

Also, Lavelle might give us the cell diagram and ask for the half-reactions, similarly to problem 14.11 in the 6th edition.
Sat Feb 23, 2019 10:26 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Closed vs Isolated
Replies: 5
Views: 424

### Re: Closed vs Isolated

viewtopic.php?f=127&t=41698
Tue Feb 12, 2019 2:59 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: 6th edition 8.25
Replies: 4
Views: 2805

### Re: 6th edition 8.25

I believe it is because the reaction is explicitly stated to release energy so -q(reaction)=-[-q(calorimeter)].
It would then be -q(reaction)=q(calorimeter).
Tue Feb 12, 2019 2:49 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Example 8.3 sixth edition
Replies: 3
Views: 149

### Re: Example 8.3 sixth edition

In the problem, it doesn't matter whether you convert it or not because you're simply finding the difference (whether you add 273 or not, the difference between the two temperatures would still be 80).
Tue Feb 12, 2019 2:39 pm
Forum: Phase Changes & Related Calculations
Topic: joules conversion factor
Replies: 7
Views: 511

### Re: joules conversion factor

It's listed on the formula sheet; just remember to convert the units to joules in the end.
Sun Feb 10, 2019 2:24 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: 6th Edition Problem 8.21
Replies: 8
Views: 287

### Re: 6th Edition Problem 8.21

Kobe_Wright wrote:What would cp be in this equation?

If the substance is anything except water, the Cp is usually given in the question.
Sun Feb 10, 2019 2:10 pm
Forum: Phase Changes & Related Calculations
Topic: Enthalpy values
Replies: 3
Views: 153

### Re: Enthalpy values

They'll be given! Here's the list of equations/values:
https://lavelle.chem.ucla.edu/wp-conten ... ations.pdf
Sun Feb 10, 2019 2:08 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Decreasing volume and chemical equilibrium
Replies: 8
Views: 331

### Re: Decreasing volume and chemical equilibrium

If you're decreasing the overall volume, the pressure will increase, shifting the reaction where there's less moles of gas.
Sun Feb 03, 2019 3:35 pm
Forum: Phase Changes & Related Calculations
Topic: Drawing a heating curve
Replies: 6
Views: 283

### Re: Drawing a heating curve

I believe the "steepness" can also depend on the intervals of your x and y axis.
Sun Feb 03, 2019 3:21 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Reversible reaction
Replies: 3
Views: 125

### Re: Reversible reaction

Here are the formulas:

"Fixed" or irreversible reactions has a constant external pressure: w=-P*deltaV
Reversible and isothermal reactions that has a changing pressure: w=-nRTln(V2/V1)
Sun Feb 03, 2019 3:17 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Week 4 HW
Replies: 2
Views: 156

### Re: Week 4 HW

Here's the syllabus for the whole list:
https://lavelle.chem.ucla.edu/wp-conten ... 4BSYLL.pdf
Sat Jan 26, 2019 7:56 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Hess' Law Confirmation
Replies: 3
Views: 119

### Re: Hess' Law Confirmation

You would cancel the reactants and products if they're on opposite sides of the equation. Make sure to remember that it should be in the same state. You might have to multiply or reverse the reaction in order to cancel some reactants/products. If you multiply, you should also do the same to the enth...
Sat Jan 26, 2019 7:48 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Hess's Law
Replies: 5
Views: 166

### Re: Hess's Law

Remember to also make sure that the states are the same if you're cancelling.
Sat Jan 26, 2019 7:42 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: method 1
Replies: 4
Views: 128

### Re: method 1

Also, remember that they have to be the same state.
Thu Jan 17, 2019 7:07 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Review of Weak Bases
Replies: 2
Views: 129

### Re: Review of Weak Bases

Apart from other guidelines, a strong base usually has (OH) in the molecular formula, such as LiOH and KOH.
Tue Jan 15, 2019 11:53 pm
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: 11.33 6th edition
Replies: 2
Views: 108

### Re: 11.33 6th edition

Similar to finding the equilibrium constant, the reaction quotient requires you to use molarity/concentration.
Tue Jan 15, 2019 11:49 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: 11.69 (6th Edition)
Replies: 3
Views: 116

### 11.69 (6th Edition)

Equation: CO(g) + H2O(g) ⇌ CO2(g) + H2(g) Question: (d) If the concentration of H2O is decreased, what happens to the equilibrium constant for the reaction? Answer: (d) The equilibrium constant for the reaction is unchanged because it is unaffected by any change in concentration. Can someone please ...
Thu Jan 10, 2019 7:57 pm
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: 11.11 (6th Edition)
Replies: 1
Views: 59

### 11.11 (6th Edition)

Can someone please explain the concepts behind 11.11c and 11.11d? I know that the coefficients are implied in part d, but I'm uncertain about the impact of it being included/omitted in the ratio and how it affects the overall value. Thanks!
Thu Jan 10, 2019 7:54 pm
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: Reaction quotient [ENDORSED]
Replies: 4
Views: 157

### Re: Reaction quotient[ENDORSED]

If you're pertaining to 11.13a (6th Edition), you use the reactants. The products, since they're solids, are simply 1 in the equation.
Thu Jan 10, 2019 7:49 pm
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: K vs Q [ENDORSED]
Replies: 7
Views: 290

### Re: K vs Q[ENDORSED]

The formula is the same. What's different between the two is the concept behind it. The equilibrium constant K is the ratio at equilibrium; it describes a reaction that is at equilibrium. On the other hand, the reaction quotient pertains to the ratio at any given time of the reaction. This might elu...
Sat Dec 08, 2018 12:17 am
Forum: Ionic & Covalent Bonds
Topic: Boiling point
Replies: 5
Views: 376

### Re: Boiling point

To sum up the previous comments, the stronger the bond, the higher the boiling point.
Sat Dec 08, 2018 12:08 am
Forum: Bronsted Acids & Bases
Topic: Bases
Replies: 4
Views: 183

### Re: Bases

Strong bases usually have [OH-] in the compound.
Sat Dec 08, 2018 12:07 am
Forum: Ionic & Covalent Bonds
Topic: Boiling point
Replies: 3
Views: 218

### Re: Boiling point

Adding to the previous comment, there are more bonds present, thus, there are more bonds to break.
Sat Dec 08, 2018 12:05 am
Forum: Properties & Structures of Inorganic & Organic Acids
Topic: strong vs weak acids
Replies: 5
Views: 213

### Re: strong vs weak acids

Strong bases usually have (OH-) in the compound.
Sat Dec 08, 2018 12:04 am
Forum: Shape, Structure, Coordination Number, Ligands
Topic: H20
Replies: 2
Views: 119

### Re: H20

H20 is neutral because if you draw its Lewis Structure, you can see that there is no formal charge on either the oxygen or the hydrogen.
Sat Dec 08, 2018 12:03 am
Forum: Naming
Topic: Naming (churro)
Replies: 3
Views: 156

### Re: Naming (churro)

Chlorine is named Chloro because it is inside the bracket; it is a ligand.
Fri Nov 30, 2018 1:52 pm
Forum: Lewis Structures
Topic: Lewis Structures for Molecules with H- AND O- atoms
Replies: 3
Views: 102

### Re: Lewis Structures for Molecules with H- AND O- atoms

If you see OH in a compound, H is usually attached to O.
Fri Nov 30, 2018 1:48 pm
Forum: Ionic & Covalent Bonds
Topic: Polar and Nonpolar
Replies: 3
Views: 117

### Re: Polar and Nonpolar

Knowing an atom's electronegativity will tell you the polarity of the molecule/compound (you will need to know the trend for this). There are exceptions though, because if there's symmetry in the molecular shape, the dipoles can "cancel" each other out. For example, for CCl4, Chlorine is m...
Fri Nov 30, 2018 1:43 pm
Forum: Dipole Moments
Topic: Lone pairs when determining hybridization
Replies: 3
Views: 100

### Re: Lone pairs when determining hybridization

You regard the pairs separately. This is used in determining the electron geometry, which then helps you acquire the hybridization of X.
Fri Nov 30, 2018 1:34 pm
Forum: Properties & Structures of Inorganic & Organic Acids
Topic: French Toast 28
Replies: 3
Views: 293

### Re: French Toast 28

In addition, Fluorine is more electronegative than Iodine; it holds on tightly to its electrons, which makes the bond shorter, and therefore stronger.
Sun Nov 25, 2018 10:46 pm
Forum: Hybridization
Topic: sigma vs. pi bonds
Replies: 4
Views: 130

### Re: sigma vs. pi bonds

Bonds-
Single: 1 sigma, 0 pi
Double: 1 sigma, 1 pi
Triple: 1 sigma, 2 pi

A sigma bond must be present if a pi bond were to exist.
Sun Nov 25, 2018 10:42 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Multiple bonds and electron density
Replies: 3
Views: 82

### Re: Multiple bonds and electron density

We don't take into account the number of bonds in finding electron density because they're connected to the same atom, thus, they are simply regarded as a single region. Below is a post similar to yours with more explanation: https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=41&t=37332 Hope th...
Sun Nov 25, 2018 10:15 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Sigma and Pi Bonding
Replies: 2
Views: 55

### Re: Sigma and Pi Bonding

Sigma bonds: end-to-end overlap and must form before a pi bond Pi bonds: side-by-side overlap and it cannot exist w/o a sigma bond To further clarify: Bonds- Single: 1 sigma, 0 pi Double: 1 sigma, 1 pi Triple: 1 sigma, 2 pi There are comments in this link that provide more clarification: https://lav...
Sun Nov 18, 2018 9:25 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Types of Bonds
Replies: 4
Views: 153

### Re: Types of Bonds

Sigma bonds: end-to-end overlap and must form before a pi bond
Pi bonds: side-by-side overlap and it cannot exist w/o a sigma bond

To further clarify:

Bonds-
Single: 1 sigma, 0 pi
Double: 1 sigma, 1 pi
Triple: 1 sigma, 2 pi
Sun Nov 18, 2018 9:20 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Octahedral to Square Planar
Replies: 7
Views: 207

### Re: Octahedral to Square Planar

If one looks at its electron geometry, it is considered to be an octahedral because we take into account the lone pairs of the central atom. However, if we look at its molecular shape, it is square planar because we only consider the bonded pairs of the central atom.

I hope this helps!
Sun Nov 18, 2018 9:16 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Water Molecule
Replies: 3
Views: 182

### Re: Water Molecule

It is helpful to know about electron geometry since the two comments above are pertaining to that. Electron Geometry: takes into account the lone pairs of the central atom (thus, it is considered to be a tetrahedral) Molecular Shape: considers bonded pairs only of the central atom (thus, its shape i...
Sun Nov 18, 2018 9:09 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Molecular Shape vs. Electron Geometry
Replies: 4
Views: 173

### Re: Molecular Shape vs. Electron Geometry

amogha_koka3I wrote:So is this an easy way to look at it: electron geometry/arrangement takes into account lone pairs of the central atom whereas molecular shape simply considers the shared bonds?

Yeah, that is the difference between the two.
Sun Nov 18, 2018 9:05 pm
Forum: General Science Questions
Topic: Thanksgiving Week
Replies: 8
Views: 408

### Re: Thanksgiving Week

There is no class on Wednesday.
Sat Nov 10, 2018 11:36 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Q. 4.1B
Replies: 1
Views: 151

### Q. 4.1B

I just wanted to verify the answer for question 4.1B. Is the reason why a linear molecule "may have lone pairs" is that said molecule can have lone pairs of electrons as long as the ends have the same amount of pairs? Or is it completely dependent on the central atom having a lone pair or ...
Sat Nov 10, 2018 11:18 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: how to know which shape it is
Replies: 6
Views: 260

### Re: how to know which shape it is

Hi!

The preceding comments are helpful, but I believe that looking at a VSEPR chart would solidify what they're saying.

This link may provide you with a better idea: https://goo.gl/images/bkB9yu

Hope this helps!
Sat Nov 10, 2018 11:08 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: trigonal pyramidal
Replies: 3
Views: 111

### Re: trigonal pyramidal

Trigonal pyramidal is applicable when there's 3 atoms bonded to a central atom that has a lone pair. It is the ideal structure because the angles are <109 degrees.
Sat Nov 03, 2018 1:58 am
Forum: Properties of Light
Topic: Standard Units
Replies: 8
Views: 336

### Re: Standard Units

Wavelength (lambda): nm or m
Frequency (nu): Hz or s^-1.
Speed of light: m/s

Hope it helps!
Sat Nov 03, 2018 1:53 am
Forum: Quantum Numbers and The H-Atom
Topic: s,p,d,f
Replies: 8
Views: 403

### Re: s,p,d,f

Also, remember after filling in the electrons in the order of 4s & 3d, you should move 3d before 4s. So, it will be 3d^n, 4s^n in your final electron configuration.
Sat Nov 03, 2018 1:45 am
Forum: Empirical & Molecular Formulas
Topic: Mass Composition
Replies: 3
Views: 184

### Re: Mass Composition

The mass composition has to add up to 100% or else you would not be able to do the problem.
Sat Nov 03, 2018 1:41 am
Forum: Significant Figures
Topic: Exam
Replies: 6
Views: 745

### Re: Exam

To provide more clarification: Addition/Subtraction : The answer should have the "smallest number of decimal places." For example: 10.2 + 1.005 + 2.35 = 13.555 = 13.6. (10.2 has one decimal present, thus, we "follow" it) 4.505 - 2.3 = 2.205 = 2.2 Multiplication/Division : One pic...
Sun Oct 28, 2018 6:30 pm
Forum: Lewis Structures
Topic: Lewis Structure Drawing
Replies: 6
Views: 189

### Re: Lewis Structure Drawing

Try to do what Lavelle teaches us. As of now, he used lines to symbolize bonds not lone electrons; dots signify electrons.
Sun Oct 28, 2018 6:25 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Subshells/Orbitals
Replies: 5
Views: 272

### Re: Subshells/Orbitals

I think we just have to know the general idea of what the f-orbital entails, not the specifics of it.
Sun Oct 28, 2018 6:19 pm
Forum: Properties of Electrons
Topic: mass of electron
Replies: 5
Views: 287

### Re: mass of electron

Yes, it is listed in the equations sheet in the front.

The link below lists all of the given:
https://lavelle.chem.ucla.edu/wp-conten ... ations.pdf

As the previous comment stated, you just have to know when to convert that to another unit.
Sun Oct 28, 2018 1:50 pm
Forum: Properties of Electrons
Topic: Shortcuts
Replies: 4
Views: 259

### Re: Shortcuts

Looking for patterns may be the way to go.

Look at how the numbers varies for n, l, ml, and ms.
Sun Oct 21, 2018 12:49 am
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Test 2 Material
Replies: 9
Views: 212

### Re: Test 2 Material

My TA said that it is all of chapter 1 and the first part of chapter 2; I don't know where the cutoff is for the latter though.
Sun Oct 21, 2018 12:46 am
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: 2.37
Replies: 2
Views: 47

### 2.37

I just wanted some clarification regarding question 2.37 part a and b because they're related in some way. Why do they mean by "electrons in the lower energy orbitals will 'shield' the electrons in the higher energy orbitals from the nucleus?" Why do electrons in the lower energy levels do...
Sun Oct 21, 2018 12:41 am
Forum: Properties of Electrons
Topic: eV to J
Replies: 7
Views: 279

### Re: eV to J

Just to verify the preceding comment, my TA said that the conversion will be listed on the test.
Sun Oct 21, 2018 12:23 am
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: 2.31
Replies: 2
Views: 137

### Re: 2.31

For (a), one must remember that subshells are the principal and angular momentum quantum numbers. The symbol for the two would be (n) and (l), with l=n-1. a.) cannot exist because if n=2, values of l can be l=1 or l=0. l=1 is the p-orbital and l=0 is the s-orbital; there is no value of l that can be...
Sat Oct 20, 2018 10:47 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: HW: 2.13
Replies: 1
Views: 30

### HW: 2.13

Can someone please explain to me the question: "Describe the orientation of the lobes of the px-, py-, and pz-orbitals with respect to the reference Cartesian axes"

I was wondering how one will depict each of the orbitals, such as the orbitals' specific characteristics if they do have one.
Tue Oct 09, 2018 3:45 pm
Forum: Significant Figures
Topic: Sig Fig zero rules
Replies: 7
Views: 756

### Re: Sig Fig zero rules

In addition, With values that end in zero but has a decimal point after it, you consider those zeros as Significant Figures. For example, 500. and 4350. has 3 and 4 Significant Figures respectively. Hope that helps! If you do have questions regarding subtraction/addition and multiplication/division ...
Mon Oct 08, 2018 7:38 pm
Forum: Significant Figures
Topic: Sig Figs: Addition and Subtraction
Replies: 3
Views: 205

### Re: Sig Figs: Addition and Subtraction

To provide more clarification: Addition/Subtraction : The answer should have the "smallest number of decimal places." For example: 10.2 + 1.005 + 2.35 = 13.555 = 13.6. (10.2 has one decimal present, thus, we "follow" it) 4.505 - 2.3 = 2.205 = 2.2 *Thus, for answers regarding subt...
Mon Oct 08, 2018 7:27 pm
Forum: Limiting Reactant Calculations
Topic: Theoretical vs Actual Yield
Replies: 5
Views: 1618

### Re: Theoretical vs Actual Yield

To add on the first reply, if the Percent Yield > 100%, the reason would be is that the product is not pure.
Fri Oct 05, 2018 12:33 am
Forum: Significant Figures
Topic: Sig Figs
Replies: 3
Views: 70

### Re: Sig Figs

To provide more clarification: Addition/Subtraction : The answer should have the "smallest number of decimal places." For example: 10.2 + 1.005 + 2.35 = 13.555 = 13.6. (10.2 has one decimal present, thus, we "follow" it) 4.505 - 2.3 = 2.205 = 2.2 Multiplication/Division : One pic...
Fri Oct 05, 2018 12:08 am
Forum: Significant Figures
Topic: Avogadro's Constant Sig Fig?
Replies: 3
Views: 73

### Re: Avogadro's Constant Sig Fig?

Just reaffirming the reply you got, the Solutions Manual goes by 6.022 x 10^23. Ergo, rounding 6.022 to 6.02 will lead to some of your answers to be off, just as you said.
Thu Oct 04, 2018 11:59 pm
Forum: Significant Figures
Topic: Sig Fig's in Problem E.21.d
Replies: 3
Views: 161

### Re: Sig Fig's in Problem E.21.d

The two precedent posts make sense, however, when one adds or subtracts, the rule for significant figures differs from the multiplication/division. When one is adding or subtracting, they should take the "smallest amount of decimal places in the data" (this can be found in one of the links...

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