QAM #4 Review

What is the molarity of 1.5 Liters of solution that contains 52 grams of LiF?

• 1.3 M

• 2.0 M

• 3.0 M

• 0.75 M

Answer explanation

To find molarity, we need to use the equation Molarity = Moles/Liters

First we need to find the moles of LiF using the mole equation:

Moles = Mass/Gram-formula mass

GFM of LiF = 7 + 19 = 26 g/mol (Periodic Table)

Moles = 52 grams/26g/mol = 2.0 moles

Now 2.0 moles/1.5 Liters = 1.3 M

What is the concentration of a solution, in parts per million, if 0.02 gram of Na₃PO₄ is dissolved in 1000 grams of water?

• 20 ppm

• 2 ppm

• 0.2 ppm

• 0.02 ppm

Answer explanation

Use the parts per million equation on the back of the reference table.

Parts per million = (mass of solute/ mass of solution) x 1,000,000

Given: 0.02 grams of solute and 1,000 grams of solution,

(0.02/1,000) x 1,000,000 = 20 ppm

Compared to a 0.1 M aqueous solution of NaCl, a 0.8 M aqueous solution of NaCl has a

• higher boiling point and higher freezing point

• higher boiling point and lower freezing point

• lower boiling point and higher boiling point

• lower boiling point and lower freezing point

Answer explanation

Recall that when something dissolves in water to form a solution, it has a few unique properties known as colligative properties.

1) Lower Freezing Point

2) Higher Boiling Point

The higher the concentration(molarity), the lower the freezing point and the higher the boiling point.

Which two compounds are electrolytes?

• C₆H₁₂O₆ and CH₃CH₂OH

• C₆H₁₂O₆ and HCl

• NaOH and HCl

• NaOH and CH₃CH₂OH

Answer explanation

Recall that an electrolyte is any substance that can form ions in water to conduct an electric current.

All ionic compounds that are soluble in water are electrolytes.

Acids and bases are also good electrolytes because they break apart in water.

All acids on Table K are electrolytes.

All bases on Table L are electrolytes except for NH₃(aq).

Given the reaction at equilibrium:

HSO₄^- + NH₃ <---> SO₄^2- + NH₄⁺

What are the two species that are acids?

• NH₃ and NH₄⁺

• NH₃ and SO₄^2-

• HSO₄^- and SO₄^2-

• HSO₄^- and NH₄⁺

Answer explanation

Recall that acids donate H⁺ and bases accept H⁺

BAAD (Bases Accept and Acids Donate)

Given the reaction:

HSO₄^- + NH₃ <--> SO₄^2- + NH₄⁺

Forwards: HSO₄^- gives an H to NH₃

Backwards: NH₄⁺ gives an H to SO₄^2-

HSO₄^- and NH₄⁺ are the acids.

When the pH of a solution is changed from 4 to 3, the hydronium ion concentration of the solution

• decreases by a factor of 10

• increases by a factor of 10

• decreases by a factor of 100

• increases by a factor of 100

Answer explanation

Recall the pH scale.

pH measures the amount of H⁺ or H₃O⁺ in water. This is referred to as hydronium ions.

pH < 7 = Acid (More H⁺ and less OH^-)

pH = 7 is Neutral (H⁺ = OH^-)

pH > 7 = Basic (Less H⁺ and more OH^-)

Recall that the pH scale goes by 10.

At pH of 4, H⁺ = 10^-4 M

At pH of 3, H⁺ = 10^-3 M

Therefore, from 4 to 3 it increases by a factor of 10.

Which indicator is blue in a solution that has a pH value of 7.0?

• bromcresol green

• methyl orange

• phenolphthalein

• thymol blue

Answer explanation

Look at Reference Table M

For any given pH range, if the pH is lower than the first number then it will be the first color. If the pH is higher than the second number then it will be the second color.

Look at bromcresol green

pH range = 3.8-5.4 (Yellow to Blue)

A pH of 7 is greater than 5.4.

Therefore, the solution will be blue.

Which equation represents a neutralization reaction?

• 6HClO --> 4HCl + 2HClO₃

• CH₄ + 2O₂ --> CO₂ + 2H₂O

• Ca(OH)₂ + H₂SO₄ --> CaSO₄ + 2H₂O

• Ba(OH)₂ + Cu(NO₃)₂ --> Ba(NO₃)₂ + Cu(OH)₂

Answer explanation

Recall that a strong acid and base will react to produce water (H₂O) and a salt (any ionic compound).

Neutralization is when H⁺ and OH^- cancel each other out.

On the left side: Look for H and OH

On the right side: Look for H₂O and a salt.

In a titration, 5.0 mL of a 2.0 M NaOH(aq) solution exactly neutralizes 10.0 mL of an HCl(aq) solution. What is the concentration of the HCl(aq) solution?

• 1.0 M

• 2.0 M

• 10 M

• 20 M

Answer explanation

Use the titration equation on the reference table.

M_AV_A = M_BV_B

Given: 10 mL of HCl(Acid) and 5.0 mL of 2.0 M NaOH(Base).

Solve for M_A

M_A x 10 mL = 5.0 mL x 2.0 M

10M_A = 10

M_A = 1.0 M

Given the potential energy diagram in the image, which numbered interval represents the activation energy of the reaction?

• 1

• 2

• 3

• 4

Answer explanation

Recall that activation energy is the amount of energy needed for a reaction to occur.

On the energy diagram, the activation energy is the distance from the start (Reactants) to the top of the hill. Do not start at 0 when looking at activation energy.