3H2 + N2 ==> 2NH3
Convert 2.90 moles N2 to moles NH3 using the coefficients in the balanced equation.
2.90 x (2 moles NH3/1 mole N2) = 2.90 x 2/1 = ??
Convert moles NH3 to grams by g = moles x molar mass.
b.1. Convert 13.75 g NH3 to moles. moles = grams/molar mass = 13.75/17 = 1.057 moles NH3.
2. Convert moles NH3 to moles H2.
1.057 moles NH3 x (3 moles H2/2 moles NH3) = 1.585 moles H2.
3. Convert moles H2 to grams. 1.585 x 2 = 3.171 g to four significant figures (the number in 13.75 in the problem. You should go back through and use the proper molar masses to four s.f. since I just used these numbers from memory.
c. Start with 8.26 x 10^-4 g H2 and calculate moles of NH3. Then remember that 1 mole NH3 will contain 6.02 x 10^23 molecules.
Post your work if you need additional assistance.