# What mass of potassium hydroxide is contained in 120 g of a 15% solution of this substance? What mass of alkali must be added

What mass of potassium hydroxide is contained in 120 g of a 15% solution of this substance? What mass of alkali must be added to this solution so that the mass fraction of potassium hydroxide becomes 25%? Given: m (solution) = 120 g, ω (KOH) = 15%, or 0.15, ω2 (KOH) = 25%, or 0.25.
Find: m (KOH) – ?, m3 (KOH) -?
Decision.
1. Calculate the mass of the solute (KOH) in the initial solution:
m (KOH) = m (solution) • ω (KOH) = 120 g • 0.15 = 18 g.
2. Calculate the mass of water in the initial solution:
m (water) = m (solution) – m (KOH) = 120 g – 18 g = 102 g.
3. Let the mass of the solute be x g, then substituting the values ​​into the formula for calculating the mass fraction ω (r.v.) = m (r.v.) / (M (r.v.) + M (water)), we obtain the algebraic equation for calculating the mass of a solute in a 25% solution:
0.25 = x / (x + 102)
0.25 • (x + 102) = x
0.25x + 25.5 = x
x – 0.25 x = 25.5
0.75 x = 25.5
x = 25.5: 0.75
x = m2 (KOH) = 34
4. Calculate the mass of the solute (KOH), which must be added to the solution:
m3 (KOH) = m2 (KOH) – m (KOH) = 34 g – 18 g = 16 g.
Answer: the mass of potassium hydroxide in a 15% solution is 18 g; it is necessary to add 16 g of potassium hydroxide to obtain a 25% solution. Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.