Calculate percentage gain and loss of mass in osmosis

The idea

Osmosis moves water across a partially permeable membrane from a dilute solution to a more concentrated one. In a mass experiment, a potato cylinder is weighed before and after soaking. Percentage change = (change in mass ÷ starting mass) × 100. If the final mass is greater, the result is a percentage gain (water entered by osmosis); if smaller, a percentage loss (water left). Example: start 2.0 g, finish 2.5 g gives a change of 0.5 g, so (0.5 ÷ 2.0) × 100 = 25% gain. Start 2.0 g, finish 1.6 g gives −0.4 g, so (0.4 ÷ 2.0) × 100 = 20% loss. Always divide by the starting mass, keep the sign or state gain/loss, and give units as %.

What earns the marks

• Percentage change = (change in mass ÷ starting mass) × 100, dividing by the original mass not the final mass.
• A gain means final mass exceeds starting mass because water entered the tissue by osmosis.
• A loss means final mass is below starting mass because water left the tissue by osmosis.
• Worked example: 2.0 g to 2.5 g is a 25% gain; 2.0 g to 1.6 g is a 20% loss.
• The answer is expressed as a percentage, with gain or loss (or sign) clearly stated.

Common mistakes and how to put them right

• Dividing by the final mass instead of the starting mass.

  Put it right: Divide the change by the starting mass.

• Forgetting to multiply by 100 and leaving a decimal.

  Put it right: Multiply by 100 to give a percentage.

• Reporting a loss as a positive gain.

  Put it right: State 'loss' or use a negative sign so direction is clear.

Exam tips

• Write the formula first, then substitute the two masses before calculating.
• Check the direction: heavier means gain, lighter means loss.
• Keep the starting mass as the denominator and show the ×100 step.