Určete hmotnost chloridu stříbrného vzniklého při srážecí reakci z 20 g dusičnanu stříbrného a 15 g chloridu sodného. Která látka je v nadbytku? Ar(Na) = 23; Ar(Cl) = 35,5; Ar(Ag) = 108
Správná odpověď je: A
Rovnice srážecí reakce vypadá takto:
AgNO3 + NaCl → AgCl + NaNO3.
Molární hmotnost AgNO3 je 170 g/mol; molární hmotnost NaCl je 58,5 g/mol; molární hmotnost AgCl je 143,5 g/mol. Nejprve musíme rozhodnout, která látka je v nadbytku. To zjistíme tak, že vypočítáme obě látková množství a porovnáme je:
n(AgNO3) = m(AgNO3)/M(AgNO3) = 20/170 = 0,118 mol
n(NaCl) = m(NaCl)/M(NaCl) = 15/58,5 = 0,256 mol.
1 mol AgNO3 odpovídá 1 molu NaCl. Látkové množství NaCl je vyšší, proto je NaCl v nadbytku.
V dalších výpočtech budeme používat látkové množství AgNO3, protože AgNO3 je méně a zreaguje beze zbytku – určuje, kolik AgCl může vzniknout. Výpočet provedeme následovně:
n(AgCl) = n(AgNO3)
m(AgCl) = n(AgCl) · M(AgCl) = [m(AgNO3)/M(AgNO3)] · M(AgCl)
m(AgCl) = (20/170) · 143,5 = 16,9 g.
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