Dr. Scientist wants to decompose acetic acid (CH3COOH) into its component elements in order to produce 42 g of hydrogen. What mass of acetic acid should he star

Question

Dr. Scientist wants to decompose acetic acid
(CH3COOH) into its component elements in
order to produce 42 g of hydrogen. What
mass of acetic acid should he start with?
1. 6.3 × 102
g
2. Not enough information is given to solve
this problem.
3. 8.4 × 102
g
4. 2.8 g
5. 0.7 g

Answer 1

The question is a little vague in stating 42 g of hydrogen is desired. Is that 42 g of elemental hydrogen (H) or 42 g of hydrogen gas (H₂).  I'll assume it is the latter (H₂ gas) since H doesn't exist for any length of time at standard temperature and pressure.

Convert to moles:  42 g H₂(g) x 1 moles H₂(g)/2.00 g = 21 moles H₂(g)

Each mole of CH₃COOH contains 2 moles H or 1 mole H₂

Therefore it will require 21 moles of CH₃COOH to produce 21 moles of H₂(g)

Converting this to mass:  21 moles CH3COOH x 60 g/mole = 1260 grams

In looking at the possible answers, it appears as though the original question actually was asking for grams of H and not H2, and that would make answer (1) 6.3x102 correct = 630 grams.  This answer is arrived at because 1 moles CH3COOH produces 2 moles H and therefore 21 moles H requires 10.5 moles CH3COOH and 11.5 moles x 60 g/mole = 630 grams.