ssNake's Chemistry [u2] Q's

[REBORN]

Decided to make a thread for [u2] chemistry - seems to be much harder than [u1]!

Q: Is it possible to have two solids formed in a precip. reaction?

[luken93]

Generally no, but feel free to find two combinations in a solubility table that will do that.

As I said though, it's usually (aq) + (aq)   ->   (s)   + (aq)

[REBORN]

A 14.2M solution of NaOH has a density of 1.42g/ml. What is the % by mass (w/w right?) of NaOH in the solution?

I got no clue :/

[b^3]

14.2M is 14.2 mol per L
lets take say 1 ml
divide it by 1000 and you get 0.0142 mol per ml
so the mass in 1 ml is m=nM which is m= 0.0142*40ish
=0.568g in 1 ml
now you know the solution, that is H₂O and NaOH is 1.42g/ml so in 1 ml there should be 1.42g
so the % by mass is the mass of NaOH over the total mass
so 0.568/1.42 *100=40% (w/w)
Is that the answer you need?

[REBORN]

Haven't been given answers yet; will get them tomorrow.

[b^3]

That is the best way that I see in doing it, but knowing me once it hits around 9-10ish, I start making stupid errors. So I hope its helps anyway.

[luken93]

That is the best way that I see in doing it, but knowing me once it hits around 9-10ish, I start making stupid errors. So I hope its helps anyway.

Looks right to me.

[epinephrine]

Yup B^3 is correct just to let you know its actually 39.96 but since it has to be to the least number of significant figures the answer is 40.0 % w/w.

[b^3]

so the mass in 1 ml is m=nM which is m= 0.0142*40ish
=0.568g in 1 ml

Yeh I was getting a bit lazy, sorry guys.

Woohoo 300th post.

[REBORN]

Calculate the density of 10M NH₃ that contains 18% NH₃ w/w.

I got an answer of d=0.944g/mL

is this right?

[REBORN]

Also - why do most ionic compounds such as Potassium Nitrate demonstrate a higher solubility at higher temps?

[b^3]

Also - why do most ionic compounds such as Potassium Nitrate demonstrate a higher solubility at higher temps?

For ionic substances to dissolve there has to be energy to break apart their crystal lattice and become ions in the solution. As you raise the temperature you are adding energy to the system, breaking parts of the lattice apart, increasing solubility. Gases are the opposite as you increase the temperature it is harder for the gas molecules to hold on to each other, as they are moving with greater kinetic energy and so don't dissolve as easily as the temperature rises.

[REBORN]

Stupid book says "C" then gives an explanation which refers to B.

Q: The mass of KNO3 in g present in 0.75L of 2.2M KNO3 sol is?

My answer = 167?

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And how do I do this?

Q: Household bleach contains 35g of NaClO per L of bleach. What vol of water must be added to 200mL of bleach to change the concentration of NaClO to 5g/L?

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And I disagree with their answer here...

Q: A lead nitrate solution has a molarity of 0.5M. How many moles of Lead nitrate in 500mL of solution?

I got 0.5?

[b^3]

n=cv
=2.2*0.75
=1.65mol
m=nM
m=1.65*(39.10+14.01+48)
=1.65*101.11
=166.8g
=167g

[b^3]

convert to M first i.e. divide by molar mass
inital concentration 35/(22.99+35.45+16)=35/74.44=0.470M
final concentration 5/74.44=0.067M
use c1v1=c2v2
v2=c1v1/c2
=0.470*0.2/0.067
=1.4L
so we need to add 1.4-0.2=1.2L to the solution

Note: I think you have to convert to M, but it may not be nessecary, can someone verify this?