Some general problems that i've had problems with for awhile now

[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) Lets 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) Lets say you put a test tube which 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, wouldnt the equilibrium shift to the reactant side, meaning few products, which means endothermic??

 

3) Lets 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-]?

[wolfson]

1) Lets 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

 

Molarity varies on volume and number of moles, the M = N / V is correct, you do not need to change that, and no dont start using other compounds, if you are given calculate number of moles of HCL you calculate that only nothing else.

 

2) Lets say you put a test tube which 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, wouldnt the equilibrium shift to the reactant side, meaning few products, which means endothermic??

 

Using thrmodynamic equations would resolve any problems, but just remeber that the equilibrium always balances, hence the word equilibrium, and also if no heat was needed to activate the reactions, and it let out heat them it would be exothermic, and opposite is endothermic.

 

3) Lets 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-]?

 

To find the [OH-] amount use the water constant Kw (1.0e-14), thus you will realise that Kw/molarity = [OH]- amount. A^2 + L(n)^k = pL

 

anymore help just email, or ask.