In 100 g of a solution of an acid with a concentration of 70%, 100 g of water was poured. What was the concentration?

In this concentration problem, you first need to find the mass of a substance in a solution. For this, we use the formula for the mass fraction (concentration) of substances:

ω = mv-va / mr-ra.

Let us derive the mass of a substance from this formula:

mv-va = ω * mr-ra.

Substitute the numerical values:

mv-va = 0.70 * 100 g = 70 g – the mass of the acid.

Now add the mass of water to the entire mass of the solution. We write all this in the denominator, and in the numerator – the mass of the acid:

ω = mv-va / (mr-pa + mwater) = 70 g / (100 g + 100 g) = 70 g / 200 g = 0.35 = 35%.

Answer: 35%.