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Answer: The number placed in front of a compound to balance a chemical reaction is called coefficient.

Explanation:

Assume this general form for a chemical equation:

  • aA + bB → cC + dD

The letters a, b, c, and d, in front of each compound A, B, C, and D, are called coefficients and indicate the number of formula units (molecules or ions) that take part in the equation.

Those coefficients are needed to balance the equation and ensure compliance with the law of mass conservation.

This example shows it:

  • Word equation: hydrogen + oxygen yields water

  • Chemical equation: H₂ (g) + O₂(g) → H₂O(g)

  • Balance, adding the coefficients so that the number of each kind of atoms is the same on the left and the right of the chemical equation:

        H₂ (g) + 2O₂(g) → 2H₂O(g)

In that equation:

  • The coefficient of H₂ (g) on the left is 1 (it is not written)
  • The coefficient of O₂(g) on the left is 2
  • The coefficient of H₂O(g) on the right is 2

You read it as: 1 mole of gaseous hydrogen and 2 moles of gaseous oxygen yield 2 moles of water vapor.

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