Answer
$0.14\space kg/m^{3}$
Work Step by Step
Here we use equation 14.8 $m=(DA\Delta C)t/L$ to find the solution.
$m=(DA\Delta C)t/L=>\frac{DA(C_{2}-C_{1})t}{L}$
$C_{1}=C_{2}-(\frac{L}{DA})(\frac{m}{t})$ ; Let's plug known values into this equation.
$C_{1}=0.28\space kg/m^{3}-\{\frac{1.9\times10^{-3}m}{(1.1\times10^{-5}m^{2}/s)(2.1\times10^{-9}m^{2})}\}(1.7\times10^{-12}kg/s)$
$C_{1}=0.14\space kg/m^{3}$
