Question

which of the following contains largest mass of hydrogen atoms? a. 5.0 moles of C2H2O4 b. 1.1 moles of C3H8O3 c. 4.0 moles of C2H2O4

Answer 1

4 answers · Chemistry 

 Best Answer

"mole" is a name for a quantity, like "dozen". It equals 6.02 x 10^23. You can talk about moles of atoms, or moles of molecules, or moles of anything, really, like eggs. But you don't actually need to know the number for this, since all we want is a relative number. Just treat it as a unit.

Now the number after the symbol for each element tells you how many atoms of that element per molecule. So 

H2O = two H atoms per molecule 
NH3 = three H atoms per molecule 
H2 = two H atoms per molecule 
CH4 = four H atoms per molecule 

Now multiply each of these by the respective number of molecules: 

H2O = two H atoms per molecule * 2 moles = 4 H moles of H atoms total 
NH3 = three H atoms per molecule * 1.5 moles = 4.5 moles of H atoms total 
H2 = two H atoms per molecule * 1 moles = 2 moles of H atoms total 
CH4 = four H atoms per molecule * 0.5 moles = 2 moles of H atoms total 

4.5 moles is the biggest number there, so B is the correct answer. 

(Why is a mole =6.02 x 10^23? That's the number of atomic mass units (AMUs) in one gram. Protons and neutrons each have a mass of one AMU. So a mole of hydrogen molecules (2 AMUs each - actually slightly higher, but not enough to bother with here) weighs 2 grams, and so on. This definition of a mole allows you to convert between mass and numbers of atoms, molecules, etc. )