Aluminum reacts with sulfuric acid to form aluminum sulfate. The chemical equation of the reaction between the two is:
`2Al + 3H_2SO_4 -> Al_2(SO_4)_3 + 3H_2`
In the reaction 2 moles of aluminum react with 3 moles of sulfuric acid. For each mole of aluminum 3/2 moles of sulfuric acid are required. The number of moles of sulfuric acid that are needed to completely react with 21 moles of aluminum is 21*(3/2) = 31.5
31.5 moles of sulfuric acid are required to completely react with 21 moles of aluminum.
First, you will need to write a balanced equation between aluminium and sulfuric acid:
2Al+3H2SO4 -> Al2(SO4)3 + 3H2
From the reaction you can deduce that two moles of Al react with three moles of H2SO4. To find out how many moles of H2SO4 reacts with 21.0 (three significant figures) of aluminium you will need to multiply 21.0 by the ratio of moles of sulfuric acid to the moles of aluminium.
So: 21.0 x 3/2 = 31.5 (three significant figures) moles of sulfuric acid.
Make sure they are to the correct number of significant figures!
First, write your balanced equation.
`"3H_2SO_4" + "2Al" -> "Al_2(SO_4)_3 + "3H_2"`
With your initial value, use stoichiometry to cancel out units. The units should cancel diagonally.
`"21molAl" = ("3molH_2(SO_4)")/("2molAl") = "31.5molH_2(SO_4)"`
Since there are three sig figs in the problem, make sure your answer also has three!
To solve the problem you'll first need a balanced equation of the reaction taking place. When sulfuric acid reacts with aluminum you should get aluminum sulfate and hydrogen gas.
2Al + 3H2SO4 --> Al2(SO4)3 + 3H2
You can look at the equation as a recipe. The recipe tells you what ingredients you need (reactants) to make your products. The recipe also tells you how much of the ingredients you need by the coefficients in front of each reactant.
So for sulfuric acid to completely react with aluminum, look at the coefficients. The ratio of sulfuric acid to aluminum is 3:2. For every 2 moles of aluminum, you need 3 moles of sulfuric acid. That means for every 1 mole of aluminum, you need 3/2 moles of sulfuric acid. So then, take the 21 moles of aluminum and multiply by 3/2 to get how many moles of sulfuric acid you need.
21 × 3/2 = 63/2 = 31.5 moles of sulfuric acid
My teacher gave us this handy Stoichiometry Chart to help us with these kinds of problems (I attached a photo). When converting from moles of one substance to another you have to multiply your known amount (21 moles) by the ratio of the coefficients. The ratio would be the coefficient of substance B (Sulfuric Acid) over the coefficient of substance A (Aluminum). To get the coefficients you have to write a balanced equation for the reaction.
I'll go ahead and give it to you; it's 2 Al + 3 H2SO4 = Al2(SO4)3 + 3 H2.
So, the coefficient ratio would be 3/2. This is what you multiply by the 21 moles of Aluminum. Your final answer is 31.5 moles of Sulfuric Acid.