PH & Concentration Help - Problem Solving

[kawaiitora]

Hi,

I need some help with a few problems. Thanks!

(1) You have a pH Electrode that reports a voltage in
mV, which is proportional to the hydrogen ion concentration
in solution. You also have pH buffers at known pH of say
4, 7 and 10. What do you do to translate the mV reading
into pH values for any given solution with pH between
4 and 10?

(2) You have DI water plus CaCl2 and NaHCO3 salts.
What do you do to prepare a solution with Ca concentration
of 10 mmol/l and total C concentration of 2 mmol/l?

 

[Hootenanny]

Hi there kawaiitora and welcome to PF,

What are your thoughts on the problems?

 

[kawaiitora]

Thanks for the quick reply. Sorry, I forgot about that.

(1) I'm thinking that I should use the Nernst equation, but I'm not sure which variation would be the best.

(2) For this one, I'm really not sure how to start. would I need to make up a molecular equation first?

 

[Hootenanny]

No problem, let's take each problem in turn.

Question One

You should first start with the basic Nernst Equation;

E = E^{\theta} - \frac{RT}{nF}\cdot\ln Q

Now, how many moles of electrons much be transferred to balance the charge on Q (which in this case is the hydrogen ion)? If we also assume standard conditions we have sorted the \frac{RT}{nF} coefficient. Now, the next thing you should do is convert the natural log into a log to the base 10; this involves a little bit of mathematics but is petty straight forward. Next, you will want to express Q in terms of hydrogen ion concentration or activity. After which, all is plain sailing . Can you go from here?

 

[kawaiitora]

Yep. Thanks, Hootenanny! Now, I need some help with the second problem...