A balanced chemical reaction gives the mole ratios between reactants and product

Question

A balanced chemical reaction gives the mole ratios between reactants and products. For example, one mole of white phosphorus, P4, reacts with five moles of oxygen, O2, to produce two moles of diphosphorus pentoxide, P2O5: P4+5O22P2O5 In this example, if we know the number of moles of phosphorus, we know that twice that much product can be made, assuming we have enough oxygen. Similarly, if we know the number of moles of oxygen, we know that two-fifths that much product can be made, assuming we have enough phosphorus. In a situation where you are given the mole amounts of both reactants, you should do this type of calculation individually for each one, finding two different values for the amount of product. The smaller of these two values is the maximum amount of product that can be made, also known as the theoretical yield. If you are given the masses of the reactants, you will first have to convert to moles. Once you have mole amounts, you can follow the procedure described above, applying the ratios shown in the balanced chemical equation.

1. What is the maximum amount in moles of P2O5 that can theoretically be made from 211 g of P4 and excess oxygen? Express your answer to three significant figures and include the appropriate units.

2.What is the maximum amount in moles of P2O5 that can theoretically be made from 240 g of O2 and excess phosphorus? Express your answer to three significant figures and include the appropriate units.

Explanation / Answer

(1)

The reaction is,

P4+5O22P2O5

Mass of P4 = 211 g

Moles of P4 = 123.90 g/mol

Moles of P4 = 211 g / 123.90 == 1.703 moles

1 mole P4 gives 2 moles P2O5

So, moles of P2O5 = 2 x 1.703 = 3.406 moles

Mass of P2O5 = 3.406 moles x molar mass

=3.406 moles x 283.89

= 967 grams

(2)

mass of O2 = 240 g

moles of O2 = 240 g / 32 g/mol == 7.5 moles

5 moles of O2 gives 2 moles of P2O5

So, 7.5 moles of O2 gives 3 moles of P2O5

So, mass of p2o5 == 3 moles x molar mass P2O5

= 3 moles x 283.89 g/mol

= 852 grams

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