Not every concentration is a molarity. Walk through any lab or read any product label and you will see percentages: 0.9% saline, 70% ethanol, 10% SDS. Percentages are quick to make and easy to communicate, but only if everyone agrees which kind of percentage is meant. This guide clears that up.
The three kinds of percentage
A percentage is parts per hundred, but the parts can be measured by mass or by volume, giving three conventions:
- % w/v (weight/volume) — grams of solute per 100 mL of solution. The most common in biology. 0.9% w/v saline is 0.9 g NaCl per 100 mL.
- % v/v (volume/volume) — millilitres of liquid solute per 100 mL of solution. Used for liquid-in-liquid mixes such as 70% v/v ethanol.
- % w/w (weight/weight) — grams of solute per 100 g of solution. Standard for concentrated acids and many industrial reagents, e.g. 37% w/w hydrochloric acid.
Making a percent solution
For % w/v, weigh the grams equal to the percentage and dissolve to 100 mL (scale up proportionally for larger volumes). For 500 mL of 10% w/v SDS, that is 50 g made up to 500 mL. For % v/v, measure the liquid volume and make up to the total — for 70% ethanol from absolute ethanol, mix 70 mL ethanol with water to 100 mL.
Percentages do not need a molar mass, which is why they are convenient for solutes whose molar mass is awkward or undefined, such as detergents and serum.
Converting percent to molarity
To convert % w/v to molarity, turn grams per litre into moles per litre by dividing by the molar mass. A 0.9% w/v NaCl solution is 9 g/L; divide by 58.44 g/mol to get 0.154 M — the familiar concentration of physiological saline. For % w/w you also need the solution density to find the mass in a given volume first.
The factor of ten converts grams-per-100-mL to grams-per-litre. The molar mass calculator supplies the MW, and the molarity calculator confirms the final figure.
Beyond percentages
For very dilute solutions, percentages become unwieldy and parts per million takes over — one ppm is roughly 1 mg per litre in water. We cover that scale in parts per million and parts per billion.
Recommended lab gear
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Volumetric Flask Set (Class A)
Class A borosilicate flasks for making solutions to an exact volume.
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