10 g H2O = ? g LiOH

Answer 1

Answer: 26.6 g of LiOH.

The piece that makes this solvable is sitting in your own comment: the reaction is CO2 + 2 LiOH -> Li2CO3 + H2O. Without a balanced equation, "10 g H2O = ? g LiOH" really is unanswerable, because grams never convert to grams directly. They convert through moles, and the mole ratio comes from the coefficients.

The three-step path, grams -> moles -> moles -> grams:

10 g H2O  x  (1 mol H2O / 18.02 g H2O)     = 0.555 mol H2O
0.555 mol H2O  x  (2 mol LiOH / 1 mol H2O) = 1.110 mol LiOH
1.110 mol LiOH  x  (23.95 g LiOH / 1 mol)  = 26.6 g LiOH

Molar masses: H2O = 2(1.008) + 16.00 = 18.02 g/mol. LiOH = 6.94 + 16.00 + 1.008 = 23.95 g/mol. The 2:1 ratio comes straight off the coefficients, 2 LiOH in front and 1 H2O behind.

A check worth doing: does the answer survive the ambiguity? The tutor who commented listed Li2O + H2O -> 2 LiOH as another possibility. Run it and see: that equation also pairs 2 LiOH with 1 H2O, so you get the same 1.110 mol and the same 26.6 g. Your answer is robust across both readings. But the third one he named, H2S + 2 LiOH -> 2 H2O + Li2S, is 2 LiOH to 2 H2O, a 1:1 ratio, and that gives only 13.3 g. So the coefficients really are the whole problem. Write the equation down before you touch the calculator.

Significant figures. "10 g" has no decimal point, so strictly it carries one significant figure and the answer rounds to 30 g. Almost every textbook intends two here, which gives 27 g. I have quoted 26.6 g so you can round it the way your course wants.

Why this reaction is worth remembering: CO2 + 2 LiOH -> Li2CO3 + H2O is the carbon dioxide scrubber used in spacecraft and submarines. Lithium hydroxide gets picked over the cheaper NaOH or KOH because lithium is so light, so per gram of scrubber you remove far more CO2, which is exactly what matters when you have to carry it into orbit. It is the reaction behind the improvised scrubber the Apollo 13 crew taped together. Read your problem backwards and it tells you something real: 10 g of water produced means roughly 24 g of CO2 was pulled out of the cabin air.

The habit to carry forward: every "grams of A to grams of B" question is the same three moves. Divide by the molar mass of A, multiply by the coefficient ratio B over A, multiply by the molar mass of B. If you cannot write the balanced equation, you cannot do the middle move, and that is the real reason this one stalled.