By using the relation,
\(m = \frac{1000 \times M}{(1000 \times d) - M \times M_{solute }}\)
Where, m = molality of the solution = 6.50 m
M = molarity of the solution
d = density = 1.89 g cm−3
Msolute = Molar mass of solute = 39 + 16 + 1 = 56 u
Substituting the values, we get
\(6.5 = \frac{1000 \times M}{1890 - M \times 56}\)
⇒ 12285 - 364 M = 1000 M
⇒ 1364 M = 12285
⇒ M = 9 mol dm-3