Question

In lab, scientists often use percentages to describe concentration. When making a solution, percentage can be expressed as weight over volume (wt/vol) or as weight over weight (wt/wt). When expressing things as %_(wt/wt) it usually assumed that 1 mL of water = 1 gram of water. This is similar to assuming molarity (mol/L) equals molality (mol/kg)

In lab, 70%_(wt/vol) ethanol is frequently used to sanitize the lab bench or research tools such as forceps. In order to produce a final volume of 150 mL of 70%_(wt/vol) ethanol, how many mL of 98%_(wt/vol) ethanol would you need to add?

The percentages can be treated as a concentration. A solution that is 70% ethanol is 70 g of ethanol and 30 mL of water, or 70 g Ethanol out of 100 mL of solution. Therefore, it is possible to use m₁v₁ = m₂v₂ and use the percentages as the concentration values.

Report your answer rounded to two decimal places

Answer 1

In order to produce a final volume of 150 mL of 70%(wt/vol) ethanol, how many mL of 98%(wt/vol) ethanol would you need to add?

m1v1 = m2v2

m1 = 98%, v1 = ?; m2 = 70%; v2 = 150 mL

v1 = m2v2 / m1 = 70% x 150 mL / 98% = 107.14 mL

107.14 mL of 98% (wt/vol) ethanol would you need to add in order to produce a final volume of 150 mL of 70%(wt/vol) ethanol.

Answer: 107.14 mL (answer rounded to two decimal places)

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