Question

Compound Z has a MW of 100. Your lab partner weighed 25 grams of compound Z and dissolved it in water to a final volume of 1 liter.
a) What is the concentration of the solution expressed as a percentage by weight (w/v)?
b) What is the concentration of the solution expressed as molarity?

Answer 1

b)moles = grams/molar mass. Solve for moles. Then M = moles/L.
a). %w/v = grams/100 mL = 25/1000 = 2.5/100 = 2.5 %w/v (but when I hear percentage by weight I think w/w.)

Answer 2

(a) to get the weight/volume percent, we just divide the mass of the compound to the volume, and multiply by 100:
25 g / 1000 mL * 100 = 2.5 % w/v

(b) recall that molarity is moles of solute per liter of solution. thus first get the number of moles of the compound by dividing the given mass by its molecular weight (MW):
25 g / 100 g/mol = 0.25 mol Z
Molarity = M = n/V = 0.25 mol / 1 L
Molarity = 0.25 M

hope this helps~ :)