(a) C$_2$H$_2$ (b) PCl$_3$
Work Step by Step
(a) There are two carbon atoms bonded to two hydrogen atoms. In hydrocarbons, the carbon atom is written first. Therefore, we have C$_2$H$_2$. (b) We have one phosphorus atom bonded to three chlorine atoms. The more electronegative element is written second. Therefore, we have PCl$_3$.