Blood has an osmotic pressure of 7.7 atm at 310 K…

Step 1. The particles. From $\pi = CRT$ we want $C$, so $C = \dfrac{\pi}{RT} = \dfrac{7.7\ \text{atm}}{0.0821\ \text{L atm/(K mol)} \times 310\ \text{K}} = 0.303\ \text{mol/L}$.

Step 2. The factor. $i = 1 + 0.90 \times (2 - 1) = 1.90$.

Step 3. The salt. Each mole of salt gives $1.90\ \text{mol}$ of particles, so the salt is $\dfrac{0.303}{1.90} = 0.159\ \text{mol/L}$.

Step 4. The mass. $0.159\ \text{mol/L} \times 58.5\ \text{g/mol} = 9.3\ \text{g/L}$.