Homework Help: Chemistry

Posted by Kimberlee on Saturday, May 15, 2010 at 2:10pm.

I have a test on monday and I NEED to ace it in order to raise my grade > . <
ldsfkjaslkf
right now, acids and bases are killing me

can somebody help me with these problems?

A buffer solution is prepared by mixing the weak base ammonia (Kb=1.77x10^-5) with ammonium chloride. The concentration of NH3 is 0.10M and that of NH4Cl is 050M. What is the pH of this solution?

If a 0.00100 mole sample of NaOH is added to 1.0L of the solution, what is the new pH of the solution?

To a new 1.0L sample of the buffer, 0.010 mole of HCl is added. What does it react with?

What happens to the pH of the same solution if another 0.10 mole of HCl is added?

Chemistry - DrBob222, Saturday, May 15, 2010 at 3:13pm
The secret to acid/base problems is to recognize what you have produced in the problem and determine the pH from that.

A buffer solution is prepared by mixing the weak base ammonia (Kb=1.77x10^-5) with ammonium chloride. The concentration of NH3 is 0.10M and that of NH4Cl is 050M. What is the pH of this solution?
Use the Henderson-Hasselbalch equation.
pH = pKa + log[(base)/(acid)]
base is NH3. acid is NH4Cl. pKa for NH3 is pKw-pKb.

If a 0.00100 mole sample of NaOH is added to 1.0L of the solution, what is the new pH of the solution?
NaOH is a strong base, it will react with the acid of the buffer.
NH4^+ + OH^- ==> NH3 + H2O
Therefore, the NH4^+ decreases due to the added NaOH and the NH3 increases. Substitute those new concns of acid/base into the HH equation and solve the for the new pH.

To a new 1.0L sample of the buffer, 0.010 mole of HCl is added. What does it react with?
Here the HCl will react with the base, NH3.
NH3 + HCl --> NH4Cl
You can see the the NH3 concn will be decreased while the NH4^+ will be increased. Substitute those into the HH equation and solve for the new pH.

What happens to the pH of the same solution if another 0.10 mole of HCl is added?
Just redo the last one.

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