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

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):

The molality is expressed in units ().

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=0.2 moles

Now, you know:

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= 20.6 grams

The answer is 20.6 grams.

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chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.