Question

Which is the mass of the solute in 400 grams of 20% solution. What is the concentration of this solution knowing that its density is
1.2 g/mL

Answer 1

Is that 20% by mass (w/w) or 20% by volume (w/v)?

I assume since it's 400 grams that 20% is 20% w/w.
2o% w/w means 20 g solute/100 g solution' therefore, 400 g will solution will be just 4 x that or
20 x (400/100) = ?

Concentration in what units. I assume you mean % w/v.
mass solution = volume x density
volume = mass/density = 400/1.2 g/mL so volume = 333 mL
80 grams solute and 333 mL solution; that is (80/333)*100 = % w/v