First, we must say what is molarity concentration:
Molarity (units mol/liter or mol/L) is defined as moles of solute in 1 liter of solution.
Here our solute is the sugar.
We have 0.51 moles and a solution of 778.91 mL
(1 L = 1000 mL), so 778.91 mL are 0.77891 L
Procedure:
0.51 moles --------- 0.77891 L
X ---------- 1 L
X = 0.654 mol/L = 0.65 mol/L
M is also used as a unit for molarity
Answer: Molarity = 0.65 mol/L