Question

90% and 97% pure acid solution are mixed to obtain 21 litres of 95% acid solution. Find the quantity of each type of acid to be mixed to form the mixture .

Answer 1

V1 = volume of 90% solution
V2 = volume of 97% solution

Volume of solution = V1 + v2 = 21 L 
Concentration = 95% = (V1 * 90% + V2 * 97%) / 21 

so  21 * 95 = 90 V1 + 97 V2
                   = 90 V1 + 97 (21 - V1) = 97 * 21 - 7 V1

So     V1 = 6 L   and V2 = 15 L

Answer 2

let the amount of 90% pure solution be x

and 97% pure solution be y (in litres)

so the total amount of acid = 21

x + y = 21 ------------ (1)

total concentration of the solution = 95%

90x + 97y = 95×21

90x + 97y = 1995 ----------- (2)

equation (1) × 97 => 97x + 97y = 2037

equation (2) × 1 => 90x + 97y = 1995

(-) (-) (-)

7x = 42

=> x = 6

from eq (1) we get,

6 + y =21

=>y = 15