Chemistry 1.48 - Reacting masses

Chemistry 1.48 - Reacting masses

Balanced equations give particle ratios, not direct gram ratios. Combine coefficients with relative formula masses, then scale the resulting mass relationship forwards or backwards.

Calculating reacting masses

A balanced equation conserves atoms, but its coefficients are not directly a
gram ratio. Different particles have different relative masses. To build a
reacting-mass ratio without using moles:

  1. Check that the equation is balanced.
  2. Calculate the relative formula mass, MrM_r, of each substance needed.
  3. Multiply each MrM_r by its coefficient in the balanced equation. These are the
    equation masses.
  4. Find the scale factor from the substance whose actual mass is given.
  5. Apply the same factor to the equation mass of the substance required.

Worked example: from one reactant to another reactant and the product

Magnesium reacts completely with oxygen:

2 Mg+OX2→2 MgO\ce{2Mg + O2 -> 2MgO}

Use Ar(Mg)=24A_r(\ce{Mg}) = 24 and Ar(O)=16A_r(\ce{O}) = 16.

SubstanceCoefficientRelative formula massEquation mass
Mg\ce{Mg}2242×24=482 \times 24 = 48
OX2\ce{O2}12×16=322 \times 16 = 321×32=321 \times 32 = 32
MgO\ce{MgO}224+16=4024 + 16 = 402×40=802 \times 40 = 80

The equation-mass ratio is therefore 48:32:8048 : 32 : 80.

If 12.0 g12.0 \,\mathrm{g} of magnesium reacts completely, the scale factor is:

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