Calculating Molarity and Molality: Tips and Tricks | Chemistry Homework Help

[Qube]

Homework Statement

Screenshot of some prof's website. This is just ridiculous.

http://i.minus.com/jnUa604Tz6bKX.png

Homework Equations

Molarity to molality.

Molality is moles / kg of solvent.

Molarity is moles / volume of solution.

The Attempt at a Solution

Pretty sure all the ones in the screenshot from the webpage of some university professor are wrong.

For 1)

a) Assume 1000 mL of solution. We therefore have 0.84 moles of sucrose (0.84 moles / 1 L = 0.84 M)

b) This means we have 1120 g of solution.

c) Solution mass = solute mass + solvent mass.

d) Solute mass = 0.84 moles * 342 g/moles = 287 grams.

e) 1120 - 287 = solvent mass = 833 g = 0.833 kg.

f) 0.84 / 0.833 isn't anywhere close to 3/4 molal solution.

 

[Lamebert]

I see nothing wrong with your math, so I guess his answer is wrong.

 

[Chestermiller]

The website incorrectly assumed that molality is the number of moles of solute per kg of solution (rather than the correct definition which you gave).

 

[Borek]

Not to mention the fact density of 0.840 M sucrose solution is 1.1079 g/mL, density of 4.91 M NaOH solution is 1.1840 g/mL, and of 0.79 M NaHCO₃ solution is 1.0455 g/mL. Neither is listed correctly (although sucrose is close).