Question

Se amesteca 300 g solutie 20 la suta cu 200 g solutie 10 la suta si o masas de apa pentru o obtine o noua solutie de concentratie procentuala 5 la suta. Masa apei folosite este:
a) 600 g
b) 950 g
c) 1100 g

Answer 1

ms1=300g
c1=20%
ms2=200g
c2=10%
c=5%
$c1 = \frac{md1}{ms1} \times 100$
$md1 = \frac{c1 \times ms1}{100} = \frac{20 \times 300}{100} = 60g$
$c2 = \frac{md2}{ms2}\times 100$
$md2 = \frac{c2 \times ms2}{100} = \frac{10 \times 200}{100} = 20g$
md=md1+md2=60+20=80g
ms=ms1+ms2=300+200=500g
$c = \frac{md}{ms + m \: apa} \times 100$
c ×(ms+m apă)=md×100
c×ms+c×m apă=md×100
c×m apă = md×100-c×ms
$m \: apa = \frac{md \times 100 - c \times ms}{c}$
$m \: apa = \frac{80 \times 100 - 5 \times 500}{5} = \frac{8000 - 2500}{5}$
$m \: apa = \frac{5500}{5} = 1100$