M KNO₃ = 101 g/mol
M H₂O = 18 g/mol
n KNO₃ = 2,5 mol
n H₂O = 10 mol
m KNO₃ = 2,5 mol * 101 g/mol =252,5 g
m H₂O = 10 mol * 18 g/mol = 180 g
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m r = m KNO₃ + m H₂O = 252,5 g + 180 g = 432,5 g
c p = m s * 100 % / m r
c p = 252,5 g * 100 % / 432,5 = 58,38 %