Chemistry Forums for Students > Undergraduate General Chemistry Forum

Concentration, Thermochem & pH problems

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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 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, 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-]?

Mitch:
1.) For every mole of Pb(NO2) it will equal 1 mol Pb which equals 2 mols of oxygen and also equals 1mol of nitrogen. How many mols of oxygen are in 1 Mol of CO2?

nemzy:
so .25 M Pb(NO2) in .00035 L has 8.75e-5 mol (.25 = mol/.00035).  So if i set up the ratios of the moles..1 mol Pb(NO2)/1 mol Pb = 8.75e-5mol/x

so solving for x i get 8.75e-5 mol then am i correct?

hmx9123:
Actually, you have it a little backward.  The capital M is molarity, mols per liter.  So, the lead nitrate solution is 0.25 mols per liter.  If you have 0.00035 liters, how many mols do you have?  You get the answer by multiplying the volume by the molarity, giving you the number of mols.  The idea behind how many mols of Pb+2 you have is that lead nitrate is soluble at room temperature and thus the cation and anion dissociate completely in water under normal conditions.  This means that for ever mol of Pb(NO3)2 that you have, you get 1 mol of Pb+2 and 2 mols of NO3- dissolved in water.

2. This question doesn't make sense.  A solid can't precipitate from a solid; there may be a solid precipitating from solution, but not solid from solid.  That's just a solid-solid reaction then.  Check the problem.

3. First, write the equation for the dissociation of HCN:

HCN --> H+ + CN-

It's already balanced, so there's no problem there.  Now, you find the hydrogen ion concentration just like you said, by using the antilog.  Since you know the hydrogen ion concentration, you can use stoichiometry to know what the cyanide ion concentration is.  For every H+ you have, you get one CN- according to the chemical equation.  Therefore, [ H+ ] = [ CN- ].

AWK:
1. Number of moles of Pb2+=0.00035x0.25
2. Melting needs always heat. After precipitation from melt this heat is evolved.
3. Concetration of H+ and CN- are the same not taking into account a dissociation of water.