Question

Which option is a solution that is 20% (v/v) methanol in water?

1.The solution contains 20 mL of methanol and 100 mL of water.

2.The solution contains 20 mL of methanol and 80 mL of water.

3.The solution contains 20 g of methanol and 100 mL of water.

4.The solution contains 20 g of methanol and 80 mL of water.

Answer 1

20 % volume/volume methanol in water contains 20 ml of methanol and 80 ml of water.

• Hence total volume of the solution= 20 + 80 = 100 ml
• In the first option total volume changes to 120 ml which is incorrect.
• In third and fourth option weight of methanol is given in gram so this is incorrect because we have to consider volume/volume.

Answer 2

Answer:

2.The solution contains 20 mL of methanol and 80 mL of water.

Explanation:

• The solution of 20% v/v methanol means that 20 units of volume of methanol is present in the 100 units volume of solution, i.e. 80 units of water is the constituent of the solution apart from methanol in the solution.

• So here accordingly we have the option of 20 mL methanol + 80 mL of water to form the solution of total 100 mL.

When a solution contains 20 mL of methanol and 100 mL of water,then the total volume of the solution is 120 mL and hence the % of v/v concentration of methanol is given as:

$M \%(v/v)=\frac{20}{120} \times 100=16.67\%$

A solution containing 20 g of methanol and 100 mL of water is of the form of % (w/v) i.e. mass of solute present in 100 unit volume of solvent to form a solution.