Chemistry Help

[MrRamon]

I recently took a quiz and couldn't figure out the first question.

4.5 g of sulfuric acid react with 2.56 g of lithium hydroxide.
How much lithium sulfate would be formed? What would be the percent yield if in a laboratory the reaction yield is 3.8 g? H2SO4 MW = 98.08 g/mol, LiOH MW = 23.95 g/mol, Li2SO4 MW = 109.94 g/mol, and H2O MW = 18.02 g/mol

H2SO4(aq) + 2LiOH(aq) to Li2SO4(aq) + 2H2O(l)

Answer key for first part of the question

(4.5g H2SO4 * (1mol H2SO4/98.08g H2SO4)) * (2mol LiOH/1mol H2SO4) * (23.95g LiOH/1 mol LiOH) = 2.2 g LiOH
.

my question is why is it showing 2.2g LiOH as the answer and not lithium suflate? Also is this the correct formula for finding the amount of lithium sulfate? I'm on spring break so I can't ask the professor. Thanks.

 

[jagec]

The answer key is calculating the stoichiometric amount of LiOH that will react with the given 4.5 g of H2SO4. It is NOT telling you how much Li2SO4 is formed.

So, what you learned from that is that the LiOH is in excess (2.2g reacts, 2.56g are in tube). You now know how many moles of Li2SO4 are formed, so you just have to calculate the mass based on its MW. After that, you take that theoretical yield (that is, after complete reaction of the limiting reactant H2SO4) compared to your actual yield to get the percent yield.

 

[MrRamon]

Thanks for the reply. The answer key did not answer the first question, but found that lithium hydroxide was in excess? Still a little confused. I know the limiting reactant is sulfuric acid which comes in at 5.0grams. so I got ((3.8/5.0) * 100) for the second part(which was correct). Ugh, brain hurts. Not sure how to arrive at the solution for how much lithium sulfate is formed.(wanted in grams)

 

[Paperdoc]

OK, the hint given showed that the limiting reactant is sulfuric acid. How many moles of that are available? 4.5 / 98.08 = 0.04588 moles (using more significant figures that are justified - will truncate later.) The reaction given says you will get the same number of moles if Li2SO4. Molecular weight of Li2SO4 is given as 109.94, so you should get 5.044 grams or, to the proper number of significant digits, 5.0 g. It appears you got that answer, so why are you confused?

As you also calculated, the Yield is 3.8 / 5.0 x 100% = 76%.

 

[Red Squirrel]

When it comes to chemistry, if all else fails, mix in more acid, and see what happens. Record the results, so it can be put on Youtube.