дано
m(H3PO4) = 9.8 g
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m(Zn3(PO4)2))-?
3Zn+2H3PO4-->Zn3(PO4)2 + 3H2↑
M(H3PO4) = 98 g/mol
n(H3PO4) = m/M = 9.8 / 98 = 0.1 mol
2n(H3PO4) = n(Zn3(PO4)2)
n(Zn3(PO4)2) = 0.1 / 2 = 0.05 mol
M(Zn3(PO4)2) = 385 g/mol
m(Zn3(PO4)2) = n*M = 0.05 * 385 = 19.25 g
ответ 19.25 г