Question

If 20 atoms of aluminum react with 45 molecules of chlorine gas, which reactant is limiting and how many more atoms/molecules would be required to use up all the reactants?

Answer 1

Al:ch2 an molar ratio.

2:3 and an x. X is 30.  20/2 = 10 so an answer is 

45/3 = 15. Both ratios are used 

45 - 30 = 15 CL2 left.

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Answer 2

Explanation:

$2Al+3Cl_2\rightarrow 2AlCl_3$

According to reaction, 2 atoms of aluminum gas reacts with 3 molecules of chlorine gas.

Then 20 atoms of aluminum will react with :

$\frac{3}{2}\times 20=30 molecules$ of chlorine

Aluminum is a limiting reagent.

Molecules of chlorine left unreacted: 45 - 30  molecules = 15 molecules

Then 15 molecules of chlorine gas will react with :

$\frac{2}{3}\times 15=10 atoms$ of aluminum

10 more atoms of aluminum will required to use up all chlorine.