Question

1) Masa de sare care trebuie adaugata la 150g de solutie de sare cu c=15% pentru a forma o solutie cu concentratia procentuala de 30% este:
a)23,41g
b)32,14g
c)26,14g
2)Se amesteca 300 g solutie 20% cu 200g solutie 10% si o masa de apa pentru a obtine o noua solutie de concentratie procentuala 5%. Masa apei folosite este:
a)600g
b)950g
c)1100g

Answer 1

1)ms=150g
c=15%
c1=30%
md2=masa de sare adăugată $c = \frac{md}{ms} \times 100 \\ md = \frac{c \times ms}{100} \\ md = \frac{15 \times 150}{100} = 22.5g$
$c1 = \frac{md + md2}{ms + md2} \times 100$
c1×(ms+md2)=100×(md+md2)
c1×ms+c1×md2=100×md+100×md2
c1×ms-100×md=100×md2-c1×md2
md2(100-c1)=c1×ms-100×md
$md2 = \frac{c1 \times ms - 100 \times md}{100 - c1}$
$md2 = \frac{30 \times 150 - 100 \times 22.5}{100 - 30} \\ \\ md2 = \frac{4500 - 2250}{70} = 32.14 g$
2)
ms1=300g
c1=20%
ms2=200g
c2=10%
c=5%
$c1 = \frac{md1}{ms1} \times 100 \\md1 = \frac{c1 \times ms1}{100} \\ md1 = \frac{20 \times 300}{100} = 60g$
$c2 = \frac{md2}{ms2} \times 100 \\ md2 = \frac{c2 \times ms2}{100} \\ md2= \frac{10 \times 200}{100} = 20g$md=md1+md2=60+20=80g
$c = \frac{md}{ms} \times 100 \\ ms = \frac{md \times 100}{c} \\ ms = \frac{80 \times 100}{5} = 1600g$ms=ms1+ms2+m apă
m apă=ms-(ms1+ms2)
m apă=1600-(300+200)=1600-500=1100g