дано
m(H2O) =100 g
m(ppaNaCL ) = 250 g
W(NaCL) = 15%
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W(NaCL) - ?
m(NaCL) = m(ppa NaCL) * W(NaCL) / 100% = 250*15% / 100% = 37.5 g
m общ(ppa NaCL) = m(H2O) + m(ppa NaCL) = 100*250 = 350 g
W(NaCL) = m(NaCL) / m общ (ppa NaCL) * 100% = 37.5 / 350 * 100% = 10.71%
ответ 10.71 %