Brushing up on some Stoichiometry

Mertzy

So I am thinking of applying for a laboratory assistant position and they had some equations available to do to show familiarity with the types of calculations they'd have me working with. I've been out of chemistry for a year or so now, and I'm wondering if I've done these right? Any help is greatly appreciated.

I'll just leave this here.

Please complete the following 3 questions. These questions are likely
real-lab problems you'll be asked to solve.

1. You need to make 500mL of a solution (Buffer A) that contains 15mM
NaCl, 10mM MOPS, and 10mM Tris. You have the following stock solutions
to work with:

4M NaCl, 100mM MOPS, and 1M Tris

How much of each solution do you use
to make Buffer A?

a. 15mM NaCl = 15*10^-3mol/L NaCl x 0.500L = 7.5*10^-3mol NaCl

4M NaCl = 4mol NaCl/L x ? = 7.5*10^-3mol NaCl, 7.5*10^-3mol NaCl/(4mol NaCl/L) = 1.9*10^-3 L of 4M NaCl

b. 10mM MOPS = 10*10^-3mol/L MOPS x 0.500L = 5.0*10^-3mol MOPS

100mM MOPS = 100*10^-3mol MOPS/L x ? = 5.0*10^-3mol MOPS, 5.0*10^-3mol MOPS/(100*10^-3mol MOPS/L) = 0.05 L MOPS

c. 10mM Tris = 10*10^-3mol/L Tris x 0.500L = 5.0*10^-3mol Tris

1M Tris = 1mol Tris/L x ? = 5.0*10^-3mol Tris, 5.0*10^-3mol Tris/(1mol Tris/L) = 5.0*10^-3mol Tris



2. How much sodium chloride do you weigh out to make 250mL of a 4M solution?

NaCl = 22.99g Na/mol NaCl+ 35.45g Cl/mol NaCl = 58.44g NaCl/mol

4M NaCl = 4mol/L x 0.250L = 1mol NaCl = 58.44g NaCl



3. How much magnesium sulfate heptahydrate do you weigh out to make 10L
of a 5mM solution?

MgSO4 = 24.31g Mg/mol MgSO + 32.07g S/mol MgSO + 4(15.99g O/mol) = 120.34g MgSO/mol

5mM MgSO = 5*10^-3mol/L x 10L = 0.05 mol MgSO = 0.05(120.34g MgSO/mol) = 6.02g MgSO

How did I do?

Rysca

:^: well done, I got the same answers

Demerdar

So, do you think you have a pretty firm grasp on the subject?

Mertzy

Generally, yes I do. The position is not in my major, but I know well enough to do what they need me to do and then some. The job is essentially the lab's bitch - mixing basic solutions, cleaning glassware, etc.

Tellerman

I'm not quite use to following calculations this way. (big fan of the T, which is hard to do online.) but did you account for the heptahydrate portion of the Magnesium sulfate?

Mertzy

Well, I sure didn't. Thanks for noticing!