Question

count the number of atoms on each side of the chemical equations and label them as equal mass or unequal mass.

Explanation:

Step1: Analyze $H_2 + O_2

ightarrow H_2O$
On the left - hand side, 2 H atoms and 2 O atoms. On the right - hand side, 2 H atoms and 1 O atom. So it's unequal mass.

Step2: Analyze $N_2+3H_2

ightarrow 2NH_3$
On the left - hand side, 2 N atoms and 6 H atoms. On the right - hand side, 2 N atoms and 6 H atoms. So it's equal mass.

Step3: Analyze $4P + 5O_2

ightarrow 2P_2O_5$
On the left - hand side, 4 P atoms and 10 O atoms. On the right - hand side, 4 P atoms and 10 O atoms. So it's equal mass.

Step4: Analyze $S_8+F_2

ightarrow SF_6$
On the left - hand side, 8 S atoms and 2 F atoms. On the right - hand side, 1 S atom and 6 F atoms. So it's unequal mass.

Answer:

$H_2 + O_2
ightarrow H_2O$: unequal mass
$N_2+3H_2
ightarrow 2NH_3$: equal mass
$4P + 5O_2
ightarrow 2P_2O_5$: equal mass
$S_8+F_2
ightarrow SF_6$: unequal mass