Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Question

Explanation / Answer

V = 50 mL of H2g is produced at STP, fin the mass of K(s) reacted

First, the reaction

K(s) + H2O(l) --> K(OH)(aq) +H2(g)

Balance it:

K(s) + H2O(l) --> K(OH)(aq) +1/2H2(g)

note that

at STP --> 1 mol will occupy 22.4 L or 22400 mL

we have only 50 mL

so

moles of H2 = 50/22400 = 0.00223214 moles of Hydrogen

so

ratio states that

1 mol of K(s) produces 1/2 moles of H2

so

0.00223214 moles of H2 --> 0.00223214*2 = 0.00446428 moles of K

mass = mol*MW = (0.00446428)(39.0983 ) = 0.17454 grams of K, 174.5 mg of K

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