Question

1. In 300g de solutie de soda se adauga 40g de soda si se obtine o solutie de concentratie 50%.Ce concentratie procentuala are prima solutie?

2.Peste 750g de solutie de c1=20%,se adauga apa pana cand concentratia devine 15%.Cata apa se adauga?

Answer 1

1)
ms=300g
md a=masa adăugată de sodă caustica
md a=40g
c1=50%
ms1=ms+mda=300+40=340g
$c1 = \frac{md1}{ms1} \times 100 \\ md1 = \frac{c1 \times ms1}{100} \\ md1 = \frac{50 \times 340}{100} = 170g$
md1=md+mda
md=md1-mda=170-40=130g
$c = \frac{md}{ms} \times 100 \\ c = \frac{130}{300} \times 100 = 43.33\%$
2)
ms1=750
c1=20
c=15%
ma=masa de apă adăugată
$c1 = \frac{md1}{ms1} \times 100 \\ md1 = \frac{c1 \times ms1}{100} \\ md1 = \frac{20 \times 750}{100} = 150g$
$c = \frac{md1}{ms1 + ma} \times 100 \\ c \times (ms1 + ma) = md1 \times 100 \\ c \times ms1 + c \times ma = md1 \times 100 \\ c \times ma = md1 \times 100 - c \times ms \\ ma = \frac{md1 \times 100 - c \times ms}{c} \\ ma = \frac{150 \times 100 - 15 \times 750}{15} \\ ma = 250 \: g$