Question

A 100 mL solution contains 5.7g MgCl2 (MM = 94 g/mol). The density of the solution is 1.03 g/mL.

Express the concentration in:

a. % by mass

b. Molarity​

Answer 1

Molarity is (wt. of solute in gm.÷molecular mass)×(1000÷volume of solution in ml.)

=(5.7g÷94gm/mol)×(1000÷100ml)=0.60638M

%by mass=(mass of solute in gm÷volume of solution in ml)×100=(5.7g÷100ml)×100=5.7

Answer 2

% by mass $= 5.53$% , Molarity $= 0.606$ moles/L

Explanation:

• Calculating mass of solution

$Density = \frac{Mass}{Volume}$

Therefore, Mass of solution $= 1.03$ g/mL × $100$ mL

Mass of solution $= 103$ g

Let

• Mass of solute represented by 'W'
• Mass of solution represented by 'S'
• Volume of solution represented by 'V'
• Molar mass of solution represented by 'M'

a. % by mass $= \frac{W}{S}$ × $100$

% by mass $= \frac{5.7}{103}$ × $100$

$= 5.53$ %

b. $Molarity = \frac{W}{M}$ × $\frac{1000}{V (mL)}$

Therefore, Molarity $= \frac{5.7}{94}$ × $\frac{1000}{100}$

$= 0.606$ moles/L