"Make a 10% solution" is not a complete instruction. Ten percent of what, by what? The answer changes the recipe.
Three kinds of percent
| Type | Means | Typical use |
|---|---|---|
| % w/v | grams per 100 mL of solution | SDS, agarose, glucose, NaCl |
| % v/v | mL per 100 mL of solution | ethanol, glycerol, Triton X-100 |
| % w/w | grams per 100 g of solution | concentrated acids and bases |
When a protocol says % with a solid, assume w/v. With a liquid, assume v/v. When it is a commercial concentrated acid, it is w/w.
% w/v in practice
- 10% SDS, 100 mL: 10 g SDS, dissolve in about 80 mL water, make up to 100 mL. Weigh it in a fume hood or with a mask. Do not autoclave SDS. Warm gently if it precipitates in the cold.
- 1% agarose gel, 100 mL: 1 g agarose in 100 mL 1X TAE or TBE. Microwave until fully clear. Water evaporates while boiling, so top up with water to the starting weight if you care about exact percentage.
- 0.9% NaCl (saline), 1 L: 9 g NaCl. That is about 154 mM.
Want more options, saved recipes and warnings? Open the full Percent-Solution Generator in BenchMate.
% v/v and 70% ethanol
For 100 mL of 70% ethanol from absolute ethanol: 70 mL ethanol, then make up to 100 mL with water. Do not just add 30 mL water. Ethanol and water shrink when mixed, so 70 + 30 gives a bit under 100 mL and a slightly higher percentage.
From 95% ethanol, use C1V1: V₁ = (70 × 100) / 95 = 73.7 mL, then make up to 100 mL.
Glycerol is so viscous that pipetting it by volume is unreliable. Weigh it instead. Glycerol density is about 1.26 g/mL, so 50 mL is about 63 g.
% w/w and concentrated reagents
Concentrated HCl is about 37% w/w with a density near 1.19 g/mL. To use it you need both numbers. That is why it gets its own row in the next section.
Percent to molarity
- 0.9% NaCl: (0.9 × 10) / 58.44 = 0.154 M.
- 37% HCl (density 1.19): (37 × 1.19 × 10) / 36.46 ≈ 12.1 M.
- 28-30% ammonia (density about 0.90): roughly 14.8 M at 28%. Check your bottle's assay.
Always add concentrated acid to water, slowly, in a fume hood. Never water to acid.
BenchMate's percent tool gives the mass or volume for any %, converts to molarity, and warns about viscous and hazardous reagents.
Frequently asked questions
What does % w/v mean?
Grams of solute per 100 mL of final solution. A 10% w/v solution has 10 g in every 100 mL.
How do I make 70% ethanol?
Measure 70 mL of absolute ethanol and make up to 100 mL with water. From 95% ethanol, use 73.7 mL and make up to 100 mL.
How do I convert percent to molarity?
For % w/v, multiply the percent by 10 and divide by the molecular weight. For % w/w, also multiply by the density in g/mL.
What molarity is 37% HCl?
About 12.1 M, using a density of 1.19 g/mL and an MW of 36.46 g/mol.
22 core bench techniques, from molarity to sterilisation. Volume 1 of The Biotechnology Techniques Bible.