Calculate the mass percent of ba(no3)2 in this solution, assuming the density of the solution is 1.000 kg/l.

You forgot to include the known characteristics of the solution.

I searched them and copy here:

volume: 1.000 liter

M = 0.0190 M

Now, you can start with the definition of mass percent.

mass percent = (grams of solute / grams of solution) * 100

grams of solute are obtained from the molar concentration:

M = (number of moles of solute) / (volume of solution in liters)

where number of moles = (grams) / (molar mass)

=> M = (grams of solute / molar mass of solute) / (volume of solution in liters)

=> grams of solute = M * (volume of solution in liters) * (molar mass of solute)

And density = (kg of solution / volume of solution in liters) =>

kg of solution = density * volume of solution in liters

grams of solution = density * (volume of solution in liters) * 1000 g/kg

=> mass percent = M * (volume of solution in liters) * (molar mass) / (density * volume of solution in liters * 1000 g/ kg) * 100

=> mass percent = M * molar mass * 10 / density

now replace the values known:

M = 0.0190 mol / liter
density = 1,000 kg / liter

molar mass of Ba(NO3)2 = 137.327 g/mol + 2*14.007 g/mol + 2*3*15.999 g/mol = 256.335 g/mol

=> mass percent = 0.0190 mol/liter * 256.335 g/mol * 10  kg/ g / (1.000 kg/liter)

=> mass percent = 48.7%