The equation for the reaction is
#M_text(r): 24.30color(white)(mmmmmmm)58.32#
#color(white)(mml)"Mg"^"2+" + 2"OH"^"-" → "Mg(OH)"_2#
#"Mass of Mg" = 0.0703 color(red)(cancel(color(black)("g Mg(OH)"_2))) × (1 color(red)(cancel(color(black)("mol Mg(OH)"_2))))/(58.32 color(red)(cancel(color(black)("g Mg(OH)"_2)))) × (1 color(red)(cancel(color(black)("mol Mg"))))/(1 color(red)(cancel(color(black)("mol Mg(OH)"_2)))) × "24.30 g Mg"/(1 color(red)(cancel(color(black)("mol Mg")))) = "0.029 29 g Mg"#
The average density of seawater is 1.025 g/mL.
∴ #"Mass of seawater" = 250 color(red)(cancel(color(black)("mL"))) × "1.025 g"/(1 color(red)(cancel(color(black)("mL")))) = "256.2 g"#
#" Mass % Mg" = ("0.029 29" color(red)(cancel(color(black)("g"))))/(256.2 color(red)(cancel(color(black)("g")))) × 100 % = "0.0114 %#
