Question 3.3.2: Balance equations. Balance the chemical equation for the neu......

You are asked to balance a chemical equation.
You are given an unbalanced equation.
Notice that both hydrogen and oxygen appear in more than one compound in the reactants. Begin by balancing the other elements, phosphorus and calcium, before balancing hydrogen and oxygen.

H$_{3}$PO$_{4}$ + Ca(OH)$_{2}$ → Ca$_{3}$(PO$_{4}$)$_{2}$ + H$_{2}$O.

$\,$             5 H                             2 H

$\,$             1 P                              2 P
$\,$             6 O                             9 O
$\,$             1 Ca                            3 Ca

Balance phosphorus. Change the coefficient in front of H$_{3}$PO$_{4}$ from 1 to 2 and note the change in H and O.

2 H$_{3}$PO$_{4}$ + Ca(OH)$_{2}$ → Ca$_{3}$(PO$_{4}$)$_{2}$ + H$_{2}$O.

$\,$             8 H                           2 H
$\,$             2 P                            2 P
$\,$             10 O                          9 O
$\,$             1 Ca                           3 Ca

Balance calcium. Change the coefficient in front of Ca(OH)$_{2}$ from 1 to 3 and note the change in H and O.

2H$_{3}$PO$_{4}$ + 3Ca(OH)$_{2}$ → Ca$_{3}$(PO$_{4}$)$_{2}$ + H$_{2}$O.

$\,$             12 H                           2 H
$\,$             2 P                              2 P
$\,$             14 O                           9 O
$\,$             3 Ca                           3 Ca

Balance hydrogen and oxygen. Changing the coefficient in front of $H_{2}O$ from 1 to 6 (each increase in the $H_{2}O$ coefficient adds two H atoms) balances both hydrogen and oxygen.

2 H$_{3}$PO$_{4}$ + 3 Ca(OH)$_{2}$ → Ca$_{3}$(PO$_{4}$)$_{2}$ + 6 H$_{2}$O
$\,$             12 H                                           12 H
$\,$             2 P                                              2 P
$\,$             14 O                                            14 O
$\,$             3 Ca                                            3 Ca
The equation is balanced.
Is your answer reasonable? When balancing equations, it is a good idea to check the atom balance for all elements in the equation when you are finished. In this case, there are equal numbers of elements in both reactants and products, so the equation is balanced.