Question

How many moles of PCl5 can be produced from 28.0 g of P4 (and excess Cl2)?

Answer 1

The balanced equation for this reaction is:

P₄ + 10 Cl_{2 -}--------> 4 PCl₅

You would usually have to determine the limiting reagent/reactant, however the problem states that there is excess Cl2 present. The limiting reactant is simply the reactant that is present in a smaller amount (fewer number of moles not grams). The amount of limiting reagent present determines the maximum amount of product that can be formed. To calculate this, you will have to use stoichiometry (using a balanced equation to relate the moles of reactant(s) needed to react with the moles of product(s) formed). Therefore, this balanced equation indicates that for every one mole of P₄ reacting with 10 moles of Cl₂, 4 moles of PCl₅ is formed.

To start, you would need to calculate the number of moles of P₄ present in order to determine the number of moles of PCl₅ formed. The mass of P₄ was given. You will need the molar mass in order to calculate the number of moles. The molar mass, simply tells you the number of grams of an element or compound in every one mole of that element or compound. This is determined by the atomic masses of each element (found on a periodic table).

Molar mass of P₄ = 4* (30.97 g/mol) = 123.88 g/mol

Calculating the number of moles:
# moles of P4 = (28.0 g) / (123.88 g/mol) = 0.226 mol P4


Now we can use the mole-to-mole ratio (coefficients) from the balanced equation to determine the moles of product that can be formed as following:

(0.226 mol P4) * (4 mol PCl5/ 1 mol P4) = 0.904 mol PCl5