The chemical formula of Glauber’s salt, used in the Kraft process, is

1. $Na_{2}CO_{3}.10H_{2}O$
2. $Na_{2}S_{2}O_{4}.2H_{2}O$
3. $Na_{2}HPO_{4}.2H_{2}O$
4. $Na_{2}SO_{4}.10H_{2}O$