# Balancing & stoichiometry lectures

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## Balancing questions » finding mass of displaced substance

1.000 g of iron filings is added to copper sufate solution. Once the copper is replaced from the solution fillings are dried and weighted - they weight 1.030 g. What mass of iron was dissolved in the solution during reaction?

As usual we will start with the reaction equation, this time in net ionic form, as it will contain all informations neede. We will also write down interesting us masses of reagents:

 Cu2+ + Fe -> Cu + Fe2+ 55.85 63.54

To solve the question we need to spot one thing - when 1 mole of iron (55.85 g) is dissolved in the solution, mass of filings increases by 63.54 - 55.85 = 7.69 g. This is crucial information that we can use to solve the question! From the question we know that filings mass increased by 0.030 g. We can set proportion:

 55.85 g : 7.69 g x g : 0.030 g

where we use fact that ratio of mass of iron dissolving in the copper solution to the increase of filings mass is constant. Once the proportions is ready we are ready too, and the final answer is

x = 0.030/7.69 × 55.85 g = 0.218 g

Mass of iron dissolved is 0.218 g.