Blend of 20% and 5% brine: new concentration

400 g of brine at 20% concentration is combined with 100 g of brine at 5% concentration. What is the concentration of the blend?

Step-by-step

1. 400 × 20% = 80
   Solute in this portion
2. 100 × 5% = 5
   Solute in this portion
3. 80 + 5 = 85
   Total solute is unchanged during mixing
4. 85 ÷ 500 = 17%
   Concentration = solute ÷ total solution

Why it works

Salt is conserved when solutions are combined: the total salt is the sum of each part's mass times its concentration, and the new concentration is that salt divided by the total mass.

Common mistakes

• Averaging 20% and 5% without allowing for the different masses.
• Dividing the salt by only one of the two masses.

Related

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