$${a = \bigg( \frac{\rho_v}{\rho} - 1 \bigg) g = \bigg( \mathrm{\frac{1200\, kg/m^3}{800\, kg/m^3} - 1\bigg) (9,8\, m/s^2) = \underline{\underline{4,9\, m/s^2}}}}$$