CHEM LAB 3-6

colligative property

A property of a solution that depends on the number, not the identity, of the solute particles.

Suppose that you add 27.8 g of an unknown molecular compound to 0.250 kg of benzene, which has a
KfKf
of 5.12oC/m. With the added solute, you find that there is a freezing point depression of 2.85oC compared to pure benzene.

What is the molar mass (in g/mol) of the unknown compound?

198.6 g/mol

Tf/Kf

What is the freezing point (in degrees Celcius) of 4.96 kg of water if it contains 211.7 g of
CaBr2CaBrX2
? The freezing point depression constant for water is 1.86 oC/m and the molar mass of
CaBr2CaBrX2
is 199.89 g/mol.

-1.125

Kfm

What mass, in grams, of NaCl needs to be added to 2.3 kg of water in order to create a solution with a freezing point of - -6.1 °C? The freezing point depression constant of water is 1.86 ºC/m.

220.3 g

Tf/Kf

If the freezing point depression for a solution is 2.5°C and Kf = 4.5°C/m, what is the molality of the solution?

0.56 m

T/Kf

If 0.98 g of an unknown was dissolved in 10.30 g of solvent and the resulting solution has a molality of 0.45 m, what is the molar mass of the unknown?

211 g/mol

mol solute/kg solvent

Glucose (molar mass=180.16 g/mol) is a simple, soluble sugar. Glucose solutions are used to treat patients with low blood sugar.
Suppose you prepare a glucose solution using the described procedure.

Step 1: Dissolve 233.9 g of glucose in enough water to make 500.0 mL of solution.
Step 2: Transfer 20.6 mL of the solution to a new flask and add enough water to make 250.0 mL of dilute solution.


What is the molar concentration of the glucose solution at the end of the procedure?

0.214

M1V1=M2V2

Suppose you are working with a NaOH stock solution but you need a solution with a lower concentration for your experiment. Calculate the volume (in milliliters) of the 1.345 M stock NaOH solution needed to prepare 250.0 mL of 0.1151 M dilute NaOH solution.

21.39 mL

M1V1=M2V2

What is the correct reading of the temperature in the pictured thermometer?

Example

44.4 C

The reaction rate is constant regardless of the amount of reactant in solution

Zero order

The reaction rate increases in direct proportion to the concentration of the reactant in solution

First order

An increase in the concentration of the reactant in solution causes the reaction rate to increase exponentially

Second order

In the Kinetics of an Iodine Clock Reaction procedure, the initial rate is calculated by multiplying the initial concentration of
_______ by half, and dividing by the elapsed time as measured with a
______

thiosulfate ion

a stopwatch

When using the method of initial rates for a kinetic study, the reaction is performed several times. One reactant concentration is kept constant, and the other varies.

several times

varies

To find the order of a reaction with respect to one reactant, you will monitor the change in rate of reaction as the concentration of the reactant is changed.

change in rate of reaction

concentration

the reactant

Which species is responsible for the blue color that appears during the iodine clock reaction?

Starch-triiodide complex

Which warning about iodine is accurate?

Iodine can stain the body and other surfaces.

Which statement is true about a chemical reaction at equilibrium?

The forward reaction rate is equal to the reverse reaction rate.

If you create an equilibrium mixture from
Fe3+FeX3+
and
SCN−SCNX−
ions, adding
Cl−ClX−
ions will shift the iron-thiocyanate equilibrium to the reactant side because it removes iron from the iron-thiocyanate equilibrium mixture.

The reaction will move backward.

Endothermic

Absorbs heat

Exothermic

Releases heat

When any reversible reaction is at equilibrium, what conditions are necessarily true?

Reactants and products are both present in the reaction mixture.

The rate of the forward reaction equals the rate of the reverse reaction.

The amounts of reactants and products has stopped changing.

The equilibria studied in the lab procedure include which two reactants?

Fe3+

[CoCl4]2-