Answer:
molality = 7.35 m
Step-by-step explanation:
To get this, let's write the expression of molality:
mo = n / kg water (1)
n is the moles of solute.
But we don't have the moles of solute, we have the %w/w of 20%. This means the following:
20 g solute -------> 100 g solution
We also has the density of 1.085 g/mL This means:
1.085 g solution ----------> 1 mL of solution.
To solve this, we need to know the volume of solution. As we don't have the volume of solution, we can assume a volume of 1 L.
With this volume we can calculate the mass of solution:
d = m/V ----> m = d*V
Solving for the mass of solution:
m = 1.085 * 1000 = 1085 g solution
Now that we have the mass of solution, we can know the mass of solute using the %w/w:
m solute = 1085 * (20/100) = 217 g of solute
We have solute, solution, let's get the solvent which is water:
mass of water = 1085 - 217 = 868 g of water or 0.868 kg of water
All we have to do now, is to get the moles. The hydrogen peroxide has the formula H₂O₂ and it's molecular weight is 34 g/mol, so the moles are:
n = 217 / 34 = 6.38 moles
Finally, the molality is:
mo = 6.38 / 0.868
mo = 7.35 m