# Calculate the amount of phosphorus (V) oxide substance, which is formed by burning 350 g

Calculate the amount of phosphorus (V) oxide substance, which is formed by burning 350 g of red phosphorus containing 10% impurities.

Given:

m (P) = 450 g

w% (impurities) = 10%

Find:

m (P2O5) -?

Solution:

4P + 5O2 = 2P2O5, – we solve the problem based on the composed reaction equation:

1) Find the mass of pure phosphorus without impurities:

if the proportion of impurities in the composition of the substance is 10%, then the proportion of pure phosphorus is 90%.

m (P) = 450 g * 0.9 = 405 g

2) Find the amount of phosphorus:

n (P) = m: M = 405 g: 31 g / mol = 13.06 mol

3) We compose a logical expression:

if 4 mol of P gives 2 mol of P2O5,

then 13.06 mol of P will give x mol of P2O5,

then x = 6.53 mol.

Answer: n (P2O5) = 6.53 mol.

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