What mass of chlorine reacts with 20.0g of iron to form iron(III) chloride? →

To use the mass of iron to find the mass of chlorine needed to react in the reaction, we have to find a way to compare them. We can do this using mole. This is because we can easily convert between moles and mass, and compare more accurately in moles. Remember, the amount of any substance that contains the same number of atoms as 12 g of carbon-12 (which can be found for any substance given the right information).

Find Mole of Iron

Moles Iron = mass ÷ molar mass

                 =  20.0g ÷ 55.84 g/mol

                 =  0.3582 moles

Determine Moles of Chlorine (Mole Ratio)

Based on the balanced equation, the mole ratio of Chlorine to Iron is 3 to 2. Therefore in the reaction, there are ³/₂ more moles of chlorine than iron.

⇒ since moles of iron =  0.3582 moles

   then moles of chlorine =  0.3582 moles × ³/₂

                                          = 0.5372

Find Mass of Chlorine

Mass chlorine = molar mass × moles

                        = 70.90 g/mol ×  0.5372 mol

                        = 38.09 g

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