Question

How many water molecules (H2O) can be produced from 6 molecules of hydrogen gas (purple) reacting with 6 molecules of oxygen gas (turquoise)? Which reactant is the limiting reactant?
Oxygen and hydrogen molecules
A. 6 water molecules, hydrogen is limiting
B. 3 water molecules, oxygen is limiting
C. 3 water molecules, hydrogen is limiting
D. 12 water molecules, oxygen is limiting

Answer 1

Hydrogen gas reacts with oxygen gas to form water according to the equation
$2H_{2(g)} + O_{2(g)} --\ \textgreater \ 2 H_{2}O_{(l)}$

From the coefficients in the reaction equation, we can see that it takes two molecules of hydrogen to react with one molecule of oxygen. Since we begin with equal amounts of both gasses, we can conclude that hydrogen is the limiting reactant because we use it up first. Thus, we use hydrogen for our stoichiometry that follows.
$(6 molecules H_{2} ) ( \frac{2 molecules H_{2}O }{2 molecules H_{2} } ) = 6 molecules H_{2}O$

Thus, the answer is B, 6 water molecules are produced with hydrogen being the limiting reactant

Answer 2

C. you can get 3 water molecules and hydrogen is the limiting factor