Calculate the molarity (M) of 450. mL (4.50 x 10^2 mL) of NaBr solution prepared by dissolving 20.50g NaBr

Question

A) 0.443 M

B) 4.34 M

C) 0.450 M

D) 4.43 M

Answer 1

Dimensional analysis conversions require for problem:

20.50g NaBr (mol / 102.894g) = 0.1992 mol NaBr

450 mL x (1L / 1000mL) = 0.45

Equation required for problem (dilution formula):

M = mol / L

M = 0.1992 / 0.45

M = 0.4427

Answer:

(A) 0.443 M

**This question really emphasizes how important it is to not round the numbers until you get to the answer. The options for the answer are very close to each other and, if you were to start rounding too early, you would most likely choose the wrong answer. I wanted to point this out as you will probably see this on an exam.

Answer 2

1. Find the molecular mass of NaBr (call this x)
2. Number of moles = 20.50/x
3. We have 20.50/x moles in 450 ml = 0.45 l
4. So in 1 l we have (1 / 0.45) x (20.50/x) moles, this is the molarity