I you add 78g KBr to a final volume of 500mL, what would the final concentration be?

Concentration of solution is number of moles per 1 L solution.

the mass of KBr added - 78 g

molar mass of KBr - 119 g/mol

number of moles of KBr added - 78 g / 119 g/mol = 0.655 mol

final volume - 500 mL

the number of moles in 500 mL - 0.655 mol

therefore in 1000 mL - 0.655 mol / 500 mL x 1000 mL/L = 1.31 mol/L

concentration of KBr is 1.31 M