For #3, write out the equation C = m / V, where m is the moles of NaCl and V is the total volume of the solution.
To find the number of moles, we'd need to refer back to question 2, which states that 1 gram of H2O is equal to 0.055508435061792 mole.
Because the question states there is 2 grams of sodium chloride, we'd multipy 2 and 0.055508435061792 to get the number of moles used.
2 x 0.055508435061792 = 0.11101687012
To get the volume of the water, you would divide the 500 mL by 1,000, to convert it to liters.
500 / 1,000 = 0.5
Your liters is 0.5.
Now, you can use c = 0.1 / 0.5
Plug in the values you found, and divide them to find the concentration of your solution.
0.1 / 0.5 = 0.25
Your concentration can either be written as 0.25 M OR 0.25 mol/L.