A 38.5 mL sample of HBr is titrated with 112 mL of 1.05 mol/L KOH. The density of the HBr solution is 1.75 g/mL. Calculate the percentage by mass of the HBr in the sample.

Question

How come we don't use density*volume to find the moles of HBr?

Answer 1

Because mols is not equal to d*v.
mass (grams) HBr soluton = density x volume = approx 67 g as the mass of that 38.5 mL sample.
How much HBr is in that 38.5 mL sample?
That's mols KOH = M x L = ?
Using the coefficients in the balanced equation, convert mols KOH to mols HBr.
Then mols HBr = grams HBr/molar mass HBr. You know molar mass and mols, solve for grams HBr in the 38.5 mL.
Then %HBr by mass = (mass HBr/mass sample)*100 = ?

mols HBr = grams HBr/molar mass HBr.