The freon with the formula $\mathrm{CCl}{}_2\mathrm{F}{}_2$ is known as

By the rule of 90, add 90 to the Freon number: $12+90=102$, giving 1 carbon, 0 hydrogen and 2 fluorine; the remaining valences are filled by $\mathrm{Cl}$, so $\mathrm{CCl}{}_2\mathrm{F}{}_2$.