Washing soda `(Na_(2)CO_(3).10 H_(2)O)` is widely used in softening of hard water. If 1 L of hard water requires 0.0286 g of washing soda, the hardness of `NaOH` in ppm is

Answer

4

Explanation

Mw of washing soda `=286`,
Mw of `NaOH=40" g mol"^(-1)`
`Ca(OH)_(2)+Na_(2)CO_(3) rarr CaCO_(3)+2 NaOH`
`0.0286 g Na_(2)CO_(3). 10 H_(2)O =0.0286/(286//2)xx10^(3)=0.2` meq
0.2 meq of `Na_(2)CO_(3)=0.2/2` mmol of `NaOH` in 1L
`=(0.1xx10^(-3))xx40g` in `10^(3)` mL.
`=0.01 g` in `10^(3)` mL
ppm `=(4xx10^(-3)xx10^(6))/10^(3)=4` ppm.