Question

what is the molarity of a solution containing 15g urea in 500cm3 of solution?

Answer 1

Hello!

We have the following data:  

M (Molarity) =? (in mol / L)

m1 (mass of the solute) = 15 g

if: 1 dm³ = 1 L

V (solution volume) = 500 cm³  → 0.5 dm³ → V (solution volume) = 0.5 L

MM (molar mass of Urea - CH4N2O)

C = 1*(12u) = 12 u

H = 4*(1u) = 4 u

N = 2*(14u) = 28 u

O = 1*(16u) = 16 u

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MM (molar mass of Urea - CH4N2O) = 12 + 4 +28 + 16 = 60 g/mol

Now, let's apply the data to the formula of Molarity, let's see:

$M = \dfrac{m_1}{MM*V}$

$M = \dfrac{15}{60*0.5}$

$M = \dfrac{15}{30}$

$\boxed{\boxed{M = 0.5\:mol/L}}\end{array}}\qquad\checkmark$

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Another way to solve is to find the number of moles (n1) and soon after finding the molarity (M), let's see:

$n_1 = \dfrac{m_1\:(g)}{MM\:(g/mol)}$

$n_1 = \dfrac{15\:\diagup\!\!\!\!\!g}{60\:\diagup\!\!\!\!\!g/mol}$

$\boxed{n_1 = 0.25\:mol}$

$M = \dfrac{n_1\:(mol)}{V\:(L)}$

$M = \dfrac{0.25\:mol}{0.5\:L}$

$\boxed{\boxed{M = 0.5\:mol/L}}\end{array}}\qquad\checkmark$

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I Hope this helps, greetings ... Dexteright02! =)