Kaplan MCAT General Chemistry Review

1) 64 g of oxygen, 14 g of nitrogen, and 66 g of carbon dioxide are confined in a container. If the total pressure is 10 atm, what is the partial pressure of each gas?

2) The partial pressure of nitrogen is 2 atm. If its mole fraction is 2/10 and the only other gas in the container is oxygen, how many moles of oxygen are in the container?

3) An unknown substance’s mole fraction is 4/10. If its partial pressure is 5 atm, what is the sum pressure of all the other gases in the container?

Normality and Molarity

Key Concepts

Chapter 9

Normality

Molarity

Oxidation and reduction

Concentration

What volume of a 2.0 M solution of lithium aluminum hydride in ether is necessary to reduce 1 mole of methyl 5-cyanopentanoate to the corresponding amino alcohol? What if a 2.0 N (with respect to H ^–) solution were used instead?

1) Determine the number of equivalents of reagent necessary to accomplish the desired transformation.

There are two functional groups that need to be reduced in the molecule, the nitrile, and the ester.

The ester will require two moles of hydride to be reduced to the alcohol.

The nitrile will also require two moles of hydride because it proceeds through an imine intermediate.

Takeaways

The molecular formula of a molecule tells you how many equivalents of a desired reagent/atom are contained within the reagent. This is why it’s important to balance reactions and draw Lewis structures correctly.

2) Compute the necessary volume of the given solution.

Don’t forget that one mole of lithium aluminum hydride contains four moles of hydride.

Things to Watch Out For

Be careful to distinguish between equivalents and moles. With molecules containing several equivalents of reagents, the number of equivalents and moles are not equal!

So we’ll need 500 mL of the 2.0 M solution.

With the 2.0 N solution, things get a little trickier. If the solution is 2.0 N with respect to H^-, that means that each liter of solution contains 2 moles of hydride, or 0.5 moles of LiAlH₄.

Remember: Normality refers to equivalents per unit volume, not necessarily moles of compound per unit volume.

Similar Questions

1) Determine the volumes necessary for the same reaction as that in the initial question if 4.0 M and 4.0 N solutions of lithium aluminum hydride were used instead.

2) If the following molecule were subjected to LiAlH₄ reduction as well, what would the product be? How much of the 2.0 M solution would be necessary? The 2.0 N solution?

3) Diisobutylaluminum hydride (DIBAL) is a common alternate hydride-reducing agent. Its structure is shown below. How much of a 2.5 M solution would be necessary to carry out the same reaction described above? A 2.5 N solution?

Molar Solubility

Key Concepts

Chapter 9

pH

Molar solubility

Common ion effect

Le Châtelier’s principle

K_sp = _[A+_]^x_{sat [B} - ]^y_sat

The molar solubility of iron(III) hydroxide in pure water at 25°C is 9.94 × 10^–10 mol/L. How would the substance’s molar solubility change if placed in an aqueous solution of pH 10.0 at 25°C?

1) Identify the balanced equation for the dissociation reaction.

The generic dissociation reaction may be expressed as follows:

A_xB_y (s) xA⁺ (aq) + yB^- (aq)

Plugging in for iron(III) hydroxide, the reaction expression is:

Fe(OH)₃(s) Fe⁺³(aq) + 30H^-(aq)

This step allows us to see how many moles of ions are added to the solution per mole dissolved.

Takeaways

The value of K_sp does not change when a common ion is present; it is a constant that is dependent on temperature. The molar solubility of the salt, however, does change if a common ion is present. To find the change in molar solubility due to the common ion effect, you must find the K_sp of the substance first.

2) Find the K_spexpression for the dissociation reaction.

Generic: K_sp = [xA⁺]^x_sat [yB^-]^y_sat

Fe(OH)₃: K_sp = [Fe⁺³] [OH^-]³

K_sp is merely an equilibrium constant, just like K, and it is given the special name “solubility product” because it tells us how soluble a solid is.

Recall that the concentrations are those at equilibrium; thus, the solution is saturated. A saturated solution contains the maximum concentration of dissolved solute.

Remember: Like all Ks, a substance’s K_spvaries only with temperature.

Things to Watch Out For