Chemistry Q's

[grami001]

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Hi everybody.... long time reader, first time posting. I had a couple of questions in Chemistry.

1. At 25C, only 1.9 g CaSO4 will dissolve in 2.00L of water. What is the equilibrium constant for the reaction below?
CaSO4 (s) <-----> Ca 2+ (aq) + SO4 2- (aq)
a. 4.9 x 10 -5
b. 1.9 x 10 -4
c. 1.4 x 10 -2
d. 7.0 x 10 -3
e. 0.90

2. A mixture of 0.200 mol NO2 and 0.200 mol CO is placed in a 1.00 L flask and given time to equilibrate. Analysis of the equilibrium mixture indicates that 0.134 mol of CO2 is present. Calculate Kc for the rxn.
NO2 (g) + CO(g) <-----> NO(g) + CO2(g)
a. 0.27
b. 0.45
c. 0.67
d. 2.0
e. 4.1

3.Nitrosyl bromide decomposes according to the chemical equation below.
2 NOBr (g) <----> 2NO (g) + Br2 (g)
When 0.260 atm of NOBr is sealed in a flask and allowed to reach equilibrium, 22% of the NOBr decomposes. What is the equilibrium constant, Kp, for the rxn?
a. 2.3x10-3
b. 4.5x10-3
c. 3.5x10-2
d. 4.8x10-2
e. 8.0x10-2

4. Which is not an amphiprotic species in water?
a. HC2O4 -
b. CH3CO2 -
c. HPO4 2-
d. H2PO4 -
e. HCO3 -

5. An aqueous solution with a pH of 10.60 is diluted from 1.0L to 1.5L. what is the pH of the diluted solution?
a. 4.84
b. 7.07
c. 9.60
d. 10.42
e. 10.78

6. Which salt forms a 0.10M aqueous solution with the lowest pH?
a. MgCO3
b. NaCl
c. NaF
d. NH4Cl
e. K3PO4

7. What is the H30+ concentration in 0.45 M HCN (aq)? (Ka of HCN = 4.0x10-10)
Answer: 1.3x10-5 (tired of typing all the answers lol)

8. The pH of aqueous 0.30 M hypochlorous acid, HClO, is 3.99. What is the pKa of this acid?
Answer: 6.54
Thanks

 

[koopa_troopa]

1. d - did 1.9/MW/volume (not sure about this, i'm not good with solutions)
2. e - use an ice table
3. e - also ice (remember stochiometry, one is a 2:1)
4. b - CH3CO2 can gain protons, not as good giving them up
5. d - I calculated pOH (this is a basic solution, so the base is being diluted.), calculate [OH-] from pOH, then diluted it. calculated new pOH and pH. not sure if this is right
6. d - all other salts are strong proton acceptors and will increase the pH
7. Ka = [H30+][CN-]/[HCN], [H3O+]=[CN-] --> solve for H3O+
8. I'm not sure, I keep getting 7.46.

I tried to lay out hints rather than details for some solutions. I hope someone can check this over as I haven't studied this since last summer. 🙂

 

[Andrew4010]

I agree with all of the answers that koopa wrote.

For 8 as well I got 7.46, Just to show you math wise what I did:

Ka = (1 x 10^-3.99)(1 x 10^-3.99)/0.3
pKa = -log(Ka)
= 7.46

Weak acid Ka calculation.. not sure how they got that answer.

I also tried another equation: 3.99 - log((10^-3.99)/0.3) ... same answer.

 

[grami001]

the answer for 8 is 7.46 lol... sorry for any confusion, i wrote the wrong answer on the question. Thanks for your replies

 

[hal9000]

Yea all of what koopa posted checks out with me. For #5 though, instead of actually doing any calculations on paper, you can do it all in your head within seconds using some deductive reasoning. You're told pH is 10.6, so this is on the 10^-10 order of magnitude in terms of concentration. Is the amount you're diluting on the order of 10s? No. It's on the order of x1.5, which is wayyy less than x10. So you can cross out a,b, and c. You're diluting a basic solution, so it'll go towards "7", and thus be less than 10.6. Which leaves d. And you're done. Don't be quick to jump to the paper and pen. 🙂

 

[cwfergus]

Can someone show me how you got d for 1?
1.9/MW/volume
gives d right. but that is the concentration is it not? and to find Ksp= [SO4][Ca]
so square it.

But thats how i thought about it, why did you just leave it as the conc'n?

 

[koopa_troopa]

Like I said, i'm not an expert on this. I think of Ksp as the maximum "concentration" of the CaSO4 that can be dissolved in solution. I tried it your way using the Ksp=[SO4][Ca] and got the answer for a. I was expecting to get the same answer. Hopefully someone can clarify this.

 

[cwfergus]

Yeah i got a) when i just tried.

Well yeah thats just how i have learnt, so hopefully someone can clarify this so i wont mess up on the mcat itself

 

[hal9000]

Yeah i got a) when i just tried.

Well yeah thats just how i have learnt, so hopefully someone can clarify this so i wont mess up on the mcat itself

The "squaring" is right. (1.9/136) = mols of CaSO4. [Ca] = [SO4] = [(mols of CaSO4)/2) = x. And Ksp = [Ca][SO4]... so Ksp = (1.9/136/2)^2 ~ 4.9*10^-5. I know I said all of what koopa posted checks out, but I didn't really do 1,2, and 3, haha. My bad. 😳

 

[bdb66]

Hey everyone, after looking over these problems i was wondering if you could tell me which chem courses this information generally applies too? I just took first semester of Gen Chem and i do not recognize any of these problems, i'm hoping they are in 2nd semes gen. and O-chem, if not then my course wasn't very thorough. Thanks in advance.

 

[cwfergus]

Hey everyone, after looking over these problems i was wondering if you could tell me which chem courses this information generally applies too? I just took first semester of Gen Chem and i do not recognize any of these problems, i'm hoping they are in 2nd semes gen. and O-chem, if not then my course wasn't very thorough. Thanks in advance.

Solubility constant, as you see in a few of them were learnt in gen chem II for me.

Most of that stuff though such as Ka and what not is from high school... atleast thats where i learnt it, havent seen it in university yet

 

[161927]

Hey everyone, after looking over these problems i was wondering if you could tell me which chem courses this information generally applies too? I just took first semester of Gen Chem and i do not recognize any of these problems, i'm hoping they are in 2nd semes gen. and O-chem, if not then my course wasn't very thorough. Thanks in advance.

Solubility chemistry, acid/base equilibria, electrochemistry, these are all 2nd semester general chemistry topics.

 

[serikk85]

Can someone show me how you got d for 1?
1.9/MW/volume
gives d right. but that is the concentration is it not? and to find Ksp= [SO4][Ca]
so square it.

But thats how i thought about it, why did you just leave it as the conc'n?

Whoa! Pretty sure the answer for 1 is not d.

Keq for this reaction is equal to Ksp

Ksp = [Ca2+][SO42-]

So to find the Ksp we need to find the concentration of each ion and multilply them together. Since CaSO4 breaks up into a 1:1 ratio of Ca2+ and SO42-. These concentrations will be equal. So we can just find the concentration of one, and then square it. So let's just find [Ca2+]

[Ca2+] will be equal to the number of moles of CaSO4 dissolved divided by liters of solution.

To find the moles of CaSO4 dissolved: 1.9g/68g/mol = .028 mol

However this is in 2 L of solution, so the molar concentration of CaSO4 is .028mol/2L = .014 mol/L.

So now we have [Ca2+] = .014

Squaring this to find Ksp = 1.9e-4

I keep getting b.

 

[cwfergus]

Whoa! Pretty sure the answer for 1 is not d.

Keq for this reaction is equal to Ksp

Ksp = [Ca2+][SO42-]

So to find the Ksp we need to find the concentration of each ion and multilply them together. Since CaSO4 breaks up into a 1:1 ratio of Ca2+ and SO42-. These concentrations will be equal. So we can just find the concentration of one, and then square it. So let's just find [Ca2+]

[Ca2+] will be equal to the number of moles of CaSO4 dissolved divided by liters of solution.

To find the moles of CaSO4 dissolved: 1.9g/68g/mol = .028 mol

However this is in 2 L of solution, so the molar concentration of CaSO4 is .028mol/2L = .014 mol/L.

So now we have [Ca2+] = .014

Squaring this to find Ksp = 1.9e-4

I keep getting b.

Not trying to judge your math, but try calculating the GAM again.
You'll see that 4 oxygens work out to ~64 , calcium ~40 and sulfur ~32g/mol

 

[serikk85]

Not trying to judge your math, but try calculating the GAM again.
You'll see that 4 oxygens work out to ~64 , calcium ~40 and sulfur ~32g/mol

Holy crap! What a rookie mistake! Guess what I used to calculate the FW? The atomic number. Wow. What an idiot. Sorry guys.

 

[cwfergus]

Holy crap! What a rookie mistake! Guess what I used to calculate the FW? The atomic number. Wow. What an idiot. Sorry guys.

Haha. thats so awesome, ive never done that before!

 

[travelbug73]

How are you all arriving at d for number 1? Is it not a = 4.9 x 10^-5?

First you get the moles
Next you get the Molarity and
Finally you square it to get the Ksp because it is an MX salt.

 

[cwfergus]

Dude look a few posts after the first,

FIRST thing i said was it was NOT d. that it was a