Question

В какой масе воды надо растворить 50 г соли,что б получить раствор с массовой долей вещества 0.2

Answer 1

Значит так. Просто пропорция.
50 г------20%
Х г-------80%(вода=100-20)

Х=50*80:20=200 г
Вот и все
P.S. Почти все в химии решается пропорцией 

Answer 2

Rastvor Mass Fraction Calculation

To calculate the mass of water needed to dissolve 50g of salt and obtain a solution with a mass fraction of 0.2, we can use the formula for mass fraction:

Mass Fraction (w) = (mass of solute) / (mass of solute + mass of solvent)

In this case, the mass fraction is given as 0.2, and the mass of the solute (salt) is 50g. We need to find the mass of the solvent (water) required to achieve this mass fraction.

Calculation

Using the formula for mass fraction, we can rearrange it to solve for the mass of the solvent:

Mass of solvent = (mass of solute) / (desired mass fraction - mass fraction)

Substituting the given values:

Mass of solvent = 50g / (0.2 - 0)

This simplifies to:

Mass of solvent = 50g / 0.2 = 250g

Therefore, to obtain a solution with a mass fraction of 0.2, 50g of salt should be dissolved in 250g of water.

Conclusion

To achieve a solution with a mass fraction of 0.2, 50g of salt should be dissolved in 250g of water.