What is mass percent of a solution?
A solution is salt (the solute) mixed evenly into water (the solvent). Some solutions are strong, some are weak. We need a number to say how strong.
The number is the mass percent (also called mass fraction of solute). It tells how many grams of salt are in every 100 g of the solution.
Mass percent = (mass of salt / mass of solution) x 100
Remember: mass of solution = mass of salt + mass of water. A 10% solution means 10 g salt in 100 g solution, so the other 90 g is water.
Step 1: Calculate the mass of salt and water
Turn the formula around:
Mass of salt = total mass x percent / 100
Mass of water = total mass - mass of salt
Example: you want 50 g of a 10% solution. Salt = 50 x 10 / 100 = 5 g. Water = 50 - 5 = 45 g.
Water has a density of 1 g/mL, so 45 g of water has a volume of 45 mL. That is why we can measure water in a cylinder instead of weighing it.
Step 2 and 3: Weigh the salt and measure the water
Weighing the salt. Switch on the balance and put a clean, dry weighing paper on the pan. Press tare (zero) so the paper counts as nothing. Use a spoon to add salt slowly until the reading is exactly the mass you calculated. Salt takes in moisture from air, so do not leave it open for long.
Measuring the water. Pour water into a measuring cylinder. When you are close, add the last few mL with a dropper. Put your eye level with the water surface and read the bottom of the curved surface (the meniscus). Do not read from above or below.
Step 4: Dissolve and stir
Pour the salt into a clean beaker. Pour in the water. Stir with a glass rod until no grains are left. The salt has split into tiny particles that spread through the water and are too small to see. Do not strike the glass wall with the rod.
Pour the solution into a labelled bottle. A good label has the name, the percent and the date, for example: NaCl solution, 10%.
Try it: your own kitchen version
If you have a kitchen scale, make 100 g of a 5% salt solution. First write the sum: salt = 5 g, water = 95 g. Weigh 5 g of salt, then add water until the total is 100 g. Never taste anything made in a lab. Now make a 2% one and compare. In the 3D, predict the numbers first, then move the sliders to check yourself.
Common errors and how to avoid them
- Forgetting to tare, so the paper's mass gets added.
- Adding the water volume to the salt without thinking: the percent is by mass.
- Leaving salt stuck on the paper or spoon: tap and brush it fully into the beaker.
- Reading the cylinder from above: you will measure too much or too little water.
- Wet spoon or wet beaker: it changes the mass of salt weighed.
Key formulas and definitions
- Mass percent = (mass of solute / mass of solution) x 100
- Mass of solute = total mass x percent / 100
- Mass of water = total mass - mass of solute
- Mass of solution = mass of solute + mass of solvent
- Water: 1 g = 1 mL (density 1 g/mL)
Worked examples
1. You want 50 g of a 10% NaCl solution. How much salt and how much water?
Salt = 50 x 10 / 100 = 5 g. Water = 50 - 5 = 45 g, which is 45 mL.
2. How much salt is needed to make 200 g of a 6% solution?
Salt = 200 x 6 / 100 = 12 g. (Water = 188 g = 188 mL.)
3. Anil dissolves 8 g of salt in 72 g of water. What is the mass percent?
Mass of solution = 8 + 72 = 80 g. Percent = 8 / 80 x 100 = 10%.
4. A doctor needs 500 g of 0.9% saline. How much salt is that?
Salt = 500 x 0.9 / 100 = 4.5 g. The rest, 495.5 g, is water.
5. A student weighs 6 g of salt by mistake and adds 45 mL of water. What percent did she really make?
Solution = 6 + 45 = 51 g. Percent = 6 / 51 x 100 = 11.8%, which is stronger than the 10% she wanted.
6. You have 100 g of a 10% solution. You add 100 g of pure water. What is the new percent?
Salt stays 10 g. New total = 200 g. Percent = 10 / 200 x 100 = 5%. Adding water makes the solution weaker.
Common mistakes
- Putting the total mass of solution as the mass of water. The water is total minus salt.
- Not pressing tare, so the paper or beaker is counted in the salt mass.
- Reading the water level from above the meniscus.
- Thinking a 10% solution means 10 g salt in 100 g of water. It means 10 g salt in 100 g of solution.