1. How many moles of ammonia NH3 are produced when 0.45 moles of nitrogen N2 reacts?

Question

N2 + 3H2 yields 2NH3

2. How many mililiters of water will be produced when 2.05 g of hydrogen reacts? The density of water is 1.00 g/mL ?
2H2 + O2 yields 2H2O

3. What is the limiting reactant when 50.0 grams of nitrogen reacts with 10.7 grams of nitrogen according to this balance equation :
N2 + 3H2 yields 2NH3

These are stoichiometry problems and I don't know how to work these out. Can you please help me ?

Answer 1

1. You can convert any mole in an equation to another in the same equation just by using the coefficients in the balanced equation.
0.45 mols N2 x (2 mols NH3/1 mol N2) = 0.45 x 2/1 = ?. This will always work. The numberator is what you want to convert to and the denominator is what you have now.

2.
A four step procedure will work almost all stoichiometry problems.
a. Write and balance the equation.
2H2 + O2 ==> 2H2O

b. Convert what you have to mols. You have grams so mols = grams/molar mass = 2.05/2 = ?

c. Use the coefficients (as I did in 1 above) to convert mols H2 to mols H2O

d. Now convert mols H2O to grams H2O. That's grams = mols H2O x molar mass H2O = ?
Note: that usually is the end of the 4-step process. In this case they want mL and give you the density to solve for that.
mass = volume x density. You have density and mass, solve for volume in mL.

3. This is a limiting reagent (LR) problem. These are just regular stoichiometry problems but done more than once in the same problem. I do these the long way. Follow the steps in #2 above and see that it is simply two stoichiometry problems rolled into one.

a. N2 + 3H2 ==> 2NH3
b1. Convert N2 to mols. mol = g/molar mass = ?
b2. Do the same for H2.

c1. Use the coefficients in the balanced equation to convert mols N2 to mols product (NH3)
c2. Do the same to convert mols H2 to mols NH3.
c3. It is likely these two values will not agree which means one of them is not right; the correct value in LR problems is ALWAYS the smaller value and the reagent producing that value is the LR.

d. The problem doesn't ask for it but if you want the grams NH3, it is the SMALLER value of mols NH3 and that x molar mass NH3.