Ca(OH)2+H2SO4=>CaSO4+H2O. 100g of sulfuric acid is reacted with 78g of calcium hydroxide. Limiting reactant, theoretical yield+% yield of H2O produced by the reactant if actual yield is 34g of H2O?

Answer 1

Always, Always, ALWAYS check if the reaction is balanced:

Ca(OH)₂ (aq) + H₂SO₄ (aq) à CaSO₄ (aq) + H₂O (l)

This one is not quite!

Balance it first:

Ca(OH)₂ (aq) + H₂SO₄ (aq) à CaSO₄ (aq) + 2H₂O (l)

That’s it for balancing.

Now the question of limiting reagent….

I always just ask myself how much product I could theoretically produce from each amount reactant. The smaller number is the LIMITING reagent.

It’s like, if I have a recipe that calls for 2 cups flour and 6 eggs. I look in my kitchen and I have 4 cups flour and 4 dozen eggs. I have enough flour to make the recipe 2 times. I have enough eggs to make the recipe (48 / 6) 8 times. I’m LIMITED by the flour. Even though I have enough eggs to make the recipe 8 times, In only have enough flour to make it 2 times. 

SO, in this case, we’re given a certain amount of calcium hydroxide and a certain amount of sulfuric acid:

78 g Ca(OH)₂ x (1 mol / 74.10 g Ca(OH)₂] x [2 mol H₂O/1 mol Ca(OH)₂] = 2.105 mol H₂O

100 g H₂SO₄ x (1 mol / 98.08 g H₂SO₄] x [2 mol H₂O/1 mol H₂SO₄] = 2.039 mol H₂O

This is a close race, but you can make slightly LESS H₂O based on the amount of H₂SO₄ present than you can based on the amount of Ca(OH)₂ present. That is, there is enough Ca(OH)₂  to make 2.1 moles of H₂O, but there is only enough H₂SO₄ to make 2.04 moles of H₂O.

Therefore the yield is LIMITED by the H₂SO₄. H₂SO₄ is the limiting reagent.

We will calculate the theoretical yield based on the LIMITING REAGENT:

Theoretically, we can make 2.039 mol H₂O, which is (2.039 * 18.00 g/mol) 36.7 grams H₂O.

I don’t know if your teacher is a stickler for significant figures, but we technically have only 1 significant figure, and the answer would be 40 grams H₂O.

Furthermore, the problem tells us you were only able to recover 34 grams of H₂O.

The percent yield is (ACTUAL YIELD/THEORETICAL YIELD) * 100

SO (34/36.7)*100 = 92.6%;

With only one significant figure, you would technically have to say 90% yield.