Answer
$8\;g$.
Work Step by Step
The given values are
Base $B=400\;mL$.
Rate $R=(1:50)100=2$.
Let the pure sodium bicarbonate is $P=x$.
The formula is
$\Rightarrow \frac{P}{B}=\frac{R}{100}$
Substitute all the values.
$\Rightarrow \frac{x}{400\;mL}=\frac{2}{100}$
Cross multiply.
$\Rightarrow 100x=800\;mL$
Divide both sides by $100$.
$\Rightarrow x=8\;mL$.
Convert $mL$ to $g$.
$\Rightarrow x=8\;mL\times \frac{1g}{1mL}$.
Simplify.
$\Rightarrow x=8\;g$.
