Some basic chemistry problems i have troube with

[nemzy]

Hi, i am a 3rd year in General Chemistry, freshmen at a university, and i have a few problems that I've encountered along the way. These questions has been bugging me for a long time now and i really hope someone can fill in the holes and gaps in for me..anyways here they are

1) Let's say you have a .00035 L volume of .25 M Pb(NO2). What are the moles of Pb2+?

This is my guess on how to do this. Since molarity = moles/volume...molarity of pb2+ = moles of pb2+/.00035 But how would i know what the molarity of Pb2+ is?? Can i just use .25 M Pb(N02) instead? Is it the same thing or am i missing something here

2) Let's say you put a test tube which has a solid in a hot bath for 30 seconds. Then you shake and stir the test tube and the solid precipitates. Is this an endo or exo thermic reaction?

My guess is that its exo, because the surroundings are hot? But i am not sure, since the precipitate dissapears, wouldn't the equilibrium shift to the reactant side, meaning few products, which means endothermic??

3) Let's say you have a pH of 0.10 M HCN solution which is 5.2. What is the [H+] and [CN-].

I know that pH = -log[H+], so to find the [H+] i would just take the antilog, but then how would i find the [CN-]?

 

[garytse86]

i thought you need the amount (moles) of Pb2+?

 

[himanshu121]

1)

Definition of Molarity = (no of moles of solute)/(Volume of solution)

Now Pb(NO2) contains equal no of moles of Pb2+ and N02-

3) use charge conservation

 

[xt]

1) "Lets say you have a .00035 L volume of .25 M Pb(NO2). What are the moles of Pb2+?"

if that is the question then i think it wants you to find the number of moles instead of molarity.

moles of Lead II Nitrite = volumn x molarity = .00035 x .25 = 8.75 x 10^-5

But since the question asked for Plumbous II (Lead) which has a valence of two plus compared to the minus one valence of Nitrite, there are therefore one Lead II per molecule of Lead II Nitrite. This means there are the same number of moles of Lead II compared to Lead II Nitrite. i.e. 1 to 1 ratio.

:. moles of Lead II = 8.75 x 10^-5

2) "Lets say you put a test tube which has a solid in a hot bath for 30 seconds. Then you shake and stir the test tube and the solid precipitates. Is this an endo or exo thermic reaction?"

The forward reaction is endothermic. If this is what you mean:

solid dissolved (aq)---> heat ---> solid precipates (s)

Remember endothermic reaction means a reaction which occurs with absorption of heat.

The back reaction is exothermic since heat must be released to the environment for the solid to dissolve again. This can be done by leaving the test tube in an environment with lower temperature than itself so that entropy is favoured for the direction of heat transfer.

3) "Lets say you have a pH of 0.10 M HCN solution which is 5.2. What is the [H+] and [CN-]."

By the Arrhenius acid and base model: Acids are chemicals which ionizes in water to produce hydrogen ions. i.e. HCN ---> Hydrogen ion + Cyanide ion.

This ionization, in this case described in the question produces equal amounts of H and CN since in the molecule of HCN the ratio of H:CN is 1:1.

Therefore if you found the moles of H, you found the mole of CN.

Hopefully this helps.