Asolution of h2so4 with a molal concentration of 5.25m has a density of 1.266 g/ml. what is the molar concentration of this solution?

So first find the moles of the H₂SO₄: Mass = Moles x RFM 
so mass = 5.25 x 98 = 514.5g of H₂SO₄

so to find how many Liters of solution use:

Volume = Density x Grams of solute (per kg +1000)

density = 1.266 x 514.5 +1000 = 1917.357kg/l

now use equation: Conc. = Moles / Volume of solution to find the conc.
                               Conc. = 5.25 x 1917.357 = 4.39Mol⁻¹
Hope that helps

Molal concentration is expressed as number of moles of solute over kg solvent while molar concentration is expressed as moles solute over liter solution.  

In this case, on a basis of 1 kg water, there are 5.25 moles * (98 g/mol) = 514.5 grams H2SO4

The liters solution is equal to
V = (514.5 g + 1000 g H2O) /(ml/1.266g) /1000 = 1.1927 L

Hence the molarity is 
5.25 moles/1.1927 L or 4.40 molar.