What is the molality of a solution that contains #"3 g"# of glucose dissolved in #"30 g"# of water?
1 Answer
Explanation:
Molality is all about moles of solute dissolved per kilogram of solvent. In other words, all you have to do in order to calculate a solution's molality is to find out how many moles of solute you have per kilogram of solvent.
Use the molar mass of glucose,
#3 color(red)(cancel(color(black)("g"))) * ("1 mole C"_6"H"_12"O"_6)/(180.16color(red)(cancel(color(black)("g")))) = "0.01665 moles C"_6"H"_12"O"_6#
Now, you know that this many moles of glucose, your solute, are dissolved in
Your goal now is to "scale up" this solution to
Convert the mass of water from grams to kilograms
#30 color(red)(cancel(color(black)("g"))) * "1 kg"/(10^3color(red)(cancel(color(black)("g")))) = "0.030 kg"#
So, if
#1 color(red)(cancel(color(black)("kg water"))) * ("0.01665 moles C"_6"H"_12"O"_6)/(0.030color(red)(cancel(color(black)("kg water")))) = "0.555 moles C"_6"H"_12"O"_6#
Therefore, you can say that the molality of the solution is
#"molality" = color(green)(|bar(ul(color(white)(a/a)color(black)("0.6 mol kg"^(-1) = "0.6 molal")color(white)(a/a)|)))#
The answer must be rounded to one sig fig, the number of sig figs given to you for the mass of glucose and the mass of water.