Question #fa222

1 Answer
Jun 22, 2017

Here's what I got.


In order to find the molality of a solution, you need to divide the total number of moles of solute by the total mass of the solvent expressed in kilograms--this will get you the number of moles of solute present in #"1 kg"# of solvent.

Use the molar mass of sodium carbonate to convert the number of grams to moles

#5.3 color(red)(cancel(color(black)("g"))) * ("1 mole Na"_2"CO"_3)/(106color(red)(cancel(color(black)("g")))) = "0.050 moles Na"_2"CO"_3#

Convert the mass of water from grams to kilograms

#90 color(red)(cancel(color(black)("g"))) * "1 kg"/(10^3color(red)(cancel(color(black)("g")))) = 9 * 10^(-2)# #"kg"#

Now divide the number of moles by the total mass of the solvent

#"molality" = "0.050 moles"/(9 * 10^(-2)color(white)(.)"kg")#

# = "0.050 moles"/(9 * 10^(-2)) * 1/"1 kg"#

# = color(darkgreen)(ul(color(black)("0.56 mol kg"^(-1))))#

I'll leave the answer rounded to two sig figs, but keep in mind that you only have one significant figure for the mass of water.