Question

What is the molarity of a 5.00x10^2 mL solution containing 21.1 g of potassium bromide (KBr)? The molar mass of KBr is 119.0 g/mol.
a)3.55x10−4M
b)42.2M
c) 0.355 M
d)8.87∗10−2

Answer 1

Calculate the number of moles (n).
                             n = (21.1 g) x (1 mol / 119 g) = 211/1190 moles
Divide this number of moles by the volume of the solution,
             M = (21/1190 moles) / (500 mL x 1L/1000mL) = 0.355 M
The answer is letter C. 

Answer 2

Volume of solution in liters:

5.00 x 10² mL / 1000 => 0.5 L 

number of moles:

mass of solute / molar mass

21.1 / 119.0 => 0.1773 moles

Molarity = number of moles / volume 

M = 0.1773 / 0.5

M = 0.355 mol/L

Answer C

hope this helps!