H2(g) + Cl2(g) 
2HCl(g)
2HCl(g)What mass of Cl2 is required to produce 2.45 g HCl?
Plan: Step 1 m(HCl) n(HCl) Step 2 n(HCl) n(Cl2) Step 3 n(Cl2) m(Cl2)
n(HCl) Step 2 n(HCl) n(Cl2) Step 3 n(Cl2) m(Cl2)Step 1: Convert the known mass of HCl to amount in moles of HCl, as this can be related to the amount in moles of Cl2 through the balanced equation for the reaction.
| n(HCl) | = | 2.45 | = 0.067123 mol |
| 36.5 |
| n(HCl) | = | m(HCl) |
| M(HCl) |
Step 2: Relate the amount in moles of the known and unknown substances using their coefficients in the balanced equation for the reaction.
| 0.0672 mol | = | n(Cl2) | n(Cl2)= 0.033562 mol | |
| 2 | 1 |
| n(HCl) | = | n(Cl2) |
| 2 | 1 |
Step 3: Calculate the mass of unknown substance using its molar mass and the amount in moles.
m(Cl2) = 0.033562 mol × 71.0 g mol–1 = 2.38 g
m(Cl2) = n(Cl2) × M(Cl2)