Help Plz! my final is tomorrow?

Question

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

Answer 1

molal means moles per kg (of solvent)

As opposed to Molarity, which is moles per litre (of solvent).

So the equation is m = moles / kg

You know m = 0.500 mol/kg

kg = 0.4000   (400.0 g water is 0.4000 kg of water)

(ps. The m for molal is usually written as a large size lower case m)

So now

0.500 (g/mol) = moles / 0.4000 (kg)

therefore moles = 0.500 (g/mol) x (0.4000 kg)

therfore moles NaBr in solution = 0.2500 moles

Now work out mass of 0.2500 moles of NaBr

moles = mass / molecular weight

So mass = moles x molecular weight

The molecular weight of NaBr = 22.99 + 79.90 = 102.89 g/mol

So mass NaBr = 0.2500mol x 102.89 g/mol

= 25.72 g

Answer 2

Find moles of water in 400 g of water

Find moles needed  of bromine for 0.5

Then multiply with molecular wieght

I guess that is the way