Asample of concentrated nitric acid has a density of 1.41g/ml and contains 70.0% hno3 by mass. what mass of hno3 is present per liter of solution.

If you assume you have just 1 liter of sample, you can calculate the mass.
1000mL x 1.4g/mL = 1410g
and 70% of that would be the nitric acid.
Multiplying 1410 by 0.7 would give you 987g, and converting that to moles would give you 15.67M.
Hope this helped! :)