Question

How many grams of sodium bromide must be dissolved in 400.0 g of water to produce a 0.500 molal solution?

Answer 1

Answer:

The answer is 20.6 grams.

Explanation:

Molality describes the concentration of a solution. It can be defined as the number of moles of a solute dissolved in 1 kilogram of solvent. Then it is equal to the moles of solute (the substance that dissolves) divided by the kilograms of solvent (the substance used to dissolve):

$Molality=\frac{number of moles of solute}{kilogram of solvent}$

The molality is expressed in units ($\frac{moles}{kg}$).

So, you can apply the following rule of three with the solution being 0.5 molal: if in 1 kg of solution there are 0.5 moles of solute, in 0.4 kg (400 g, being 1kg = 1000g) how many moles of solute are there?

$moles=\frac{0.4 kg*0.5 moles}{1 kg}$

moles=0.2 moles

Now, you know:

• Na: 23 g/mole
• Br: 80 g/mole

Then, The molar mass of sodium bromide NaBr is

NaBr= 23 g/mole + 80 g/mole= 103 g/mole

Now a new rule of three applies, if in 1 mole of sodium bromide there are 103 grams, in 0.2 mole how much mass is there?

$mass=\frac{0.2 moles*103 grams}{1 mole}$

mass= 20.6 grams

The answer is 20.6 grams.