Question

if a flask of 750 ml of 0.80m of nacl solution was left, and 275 ml evaporated, what is the new concentration

Answer 1

Since no NaCl was lost, we can use
C1V1=C2V2
C1=0.8 M
V1=750 mL
V2=(750-275)=475 mL
So
C2=C1V1/V2
=0.8*750/475
=1.26 mL

Note: I provide an extra significant figure when the first digit is "1". Your teacher may or may not recommend this.

Answer 2

I want to point out to you that you need to find the caps key on your keyboard and use it. You may (or may not) know that m stand for molality and M stands for molarity. Two separate animals. My assumption is that you meant M = molarrity and MathMate worked it that way, also. I also note that MathMate made a typo. That's 1.26 M for the concentration of the resulting solution.

Answer 3

Thank you for pointing out the oversight, and the delicate case difference!