Some Chemistry Help Please, I'm Confused...?

Question

I'm usually pretty good in my chemistry lab but this stuff confuses me. I would appreciate if someone could show me how to do it, not just give answers, that way I'll know how to do my next pre-lab, because they build on each other. I'll give you best answer if I understand, thanks! What is:

1.) the mass of potassium hydrogen phthalate for 10mL of 0.2M NaOH

2.) the volume of pure HOAc to make 50mL of 0.05M HOAc.

Any help will be appreciated....thanks again.

Answer 1

1) 10 ml of 0.2M NaOH is 2 mmoles, so you will need 2 mmoles of KHP, which weighs 204.23 g/mol, or 0.408 g.

2) Pure HOAc is glacial acetic acid, which is 100% by weight acetic acid. If you look up its density, it's 1.05 g/ml, so a liter of it weighs 1050 g. The MW is 60, so there are 17.5 moles in one liter, for a concentration of 17.5 M.

So, if you need to make 50 ml of 0.05M acetic acid:

Vc = Md * Vd / Mc = 0.05 * 50 / 17.5 = 0.143 mL

In practicality, unless you're working with small volume pipettors, you would do serial dilutions to get there; i.e. make up a 1 M solution from the concentrated, then make the 0.05 M solution from the 1 M solution.

Answer 2

First work out the moles: volume x molarity/100

So your first one would be 10 x 0.2/1000.

The first one is a 1:1 mole ratio, so the moles of KHP is going to be exactly the same. So now just simply multiply by the molecular weight (we call it Mr in the UK).

Do the same for the second problem. Work out the moles, and multiply by the Mr. That will give you the mass. You will need to find out the density. Then vol = mass/density.

Answer 3

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