# Calculating molarity.... am I doing it right?

Asked by prioritymail (1630) September 24th, 2011

The question is:

If the conc of a L of soln is 2 ppm and the mass wt of chemical compound is 300 g, what is the molarity?

What I’m thinking:

Molarity = mol solute / L solution (by definition)
1 mol of chemical compound = 300 g (by definition)
ppm = mg / L (by definition)

Therefore,

2 ppm = x mg / L; x = 2 mg
mol solute = 2 mg * (mol / 300 * 10^6 mg) = 6.67 * 10^6 mol
Molarity = 6.67 * 10^-6 M

Now this seems like a ridiculously small number for an answer to a homework problem. I’m wondering if I did something wrong, particularly regarding extracting the mg of compound from given ppm solution concentration. Thanks in advance for help!

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I’m sorry, I’m having trouble following your solution with your abbreviations. What do you mean by mg? Milligrams? Moles per gram? Grams per mole? What do you mean by mass wt? Mass weight? What does that mean? Do you mean molecular weight?

Sorry, I haven’t done chemistry for a while.

shrubbery (10311)

mg = milligrams (I thought this was standard shorthand?)
mass wt = mass weight = what is written on my handout by the professor but I’m sure he means molecular weight

@blueiiznh I’m not asking you to do my homework. I already did it. I’m just asking if my process is correct or not. I learned this chemistry stuff 10 years ago and it’s tough to remember.

prioritymail (1630)

Oh well, it looks right to me… I don’t really know what “mass weight” means here as to me (physics background) that is an oxymoron. I’m submitting as is… it’s not even going to be graded.

prioritymail (1630)

@prioritymail you’re right mg is standard for milligrams but I thought ppm was in grams per litre and that is why I was confused. Don’t mind me though haha.

shrubbery (10311)

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