Question

30gm of nacl dissolved in 350g of solvent . calculate molality​

Answer 1

Solution:

Given , 30g of solution dissolved in 350g of solvent . Here , we need to find molality

We know that

Molality = Number of moles of solute/mass of solvent in kg

★ Conversion:

Here , given 30g of NaCl

Firstly , finding mass of NaCl

NaCl = 1 × (23) + 1 × (35.5)

NaCl = 23 + 35.5

NaCl = 58.5

That means , if NaCl has a mass of 58.5g then it has 1 mole

In other form:

58.5g of NaCl → has 1 mole

For 30g → ? moles

By cross multiplication

30 × 1 = 58.5 × ?

? = 30/58.5

? = 0.51

Hence , number of moles of NaCl = 0.51 moles. That means no. of moles of solute = 0.5 moles

Now , coverting 350g → kg

To covert we have to multiply with 10-³

350 × 10-³

350/10³

35/100

0.35 kg

Hence , 350g = 0.35kg . That means mass of solvent in kg = 0.35kg

______________________________

Now , coming to the question

Molality = No. of moles of solute/Mass of solvent kg

Molality = 0.51 ÷ 0.35

Molality = 1.45 mol kg-¹

Hence , molality of the solution = 1.45 mol/kg