How to make a buffer?

Question

So I have to make a 0.02 M Sodium Phosphate buffer @ pH of 7, easy enough I use monosodium and disodium phosphate as the acid and base, I used the henderson hasselbach equation. But now I have to make a 0.02 M Sodium phosphate buffer with 1 M NaCl?? I'm not sure what this means. When I asked the professor, she said that we have to have these two different solutions in one beaker and have the pH at 7. I need .75 ml of this solution. How can I accomplish this?

Answer 1

If you know how to make the buffer, and it sounds like you do, then you just use that buffer as the solvent to dissolve the solute NaCl.

Is the final solution supposed to be pH 7 or 7.1?  In your question you state the buffer is pH 7, but then state the two solutions in the beaker should be pH 7.1.  This doesn't make sense.  I'll assume you want the pH to be 7.

 

To make 1 M NaCl, you need 58.44 g NaCl/Liter of solution.

To make 10 mls, you need 58.44 g/L x 0.01 L = 0.5844 g NaCl

So, you would weigh out 0.58 g NaCl and dissolve in enough 0.02 M sodium phosphate buffer to make a final volume of 10 mls.  Then use 0.75 ml of this solution.  The pH should be the same as the pH of the original buffer since NaCl is a neutral salt.