Unit 7 - Solution Chemistry - Honors Addendum

Question 1

Definition of Net Ionic Equation (3)

Classification, Action and Goal

Answer 1

1. Chemical Reaction of Aqueous Solution and Ions
2. Excludes Spectator Ions (to show which ions participate in rxn)
3. Predicts formation of Precipitate in Double Replacement Rxn

Question 2

Definition of Spectator Ions

Answer 2

Ions in a chemical reaction that do not undergo any change

Question 3

How do you Solve a Net Ionic Equation? (6)

Answer 3

1. Predict Products of Rxn (Double Replacement of Anions & Cations)
2. Balance Ionic Equation
3. Dissociate Ions (aq) to Cations & Anions (Insoluble Copounds (s) CANNOT Dissociate)
4. Add Charges & Phases - (aq) for dissociated Ions and (s) for insoluble compounds
5. Cross Out Spectator Ions that Dissociated & didn’t change charge/phase
6. NEVER CROSS OUT SOLID INSOLUBLE COMPOUNDS IN REACTANT

Question 4

Definition of Molality

Answer 4

Moles of Solute/Kilograms of Solvent

Question 5

Why do we use Molality instead of Molarity? (3)

Answer 5

1. Solute affects the Total Volume of Solution
2. We want to know the condition BEFORE it dissolves
3. So, we measure the solven in mass instead of volume

Question 6

What is Van Hoff’s Equation on Boiling Point Elevation and Freezing Point Depression

Answer 6

Boiling Point Elevation: Δ𝑇𝑏= 𝑖 ⋅ 𝐾𝑏 ⋅ 𝑚
Freezing Point Depression: Δ𝑇𝑓 = 𝑖 ⋅ 𝐾𝑓 ⋅𝑚

i = Number of moles of ions resulting from dissociation
𝐾𝑏 = Molal BP Elevation Constant of Solvent
𝐾𝑓 = Molal FP Depression Constant of Solvent
𝑚 = Concentration of Solution Expressed in Molality
Δ𝑇𝑏 = Change in BP of Solvent
Δ𝑇𝑓 = Change in FP of Solvent

Question 7

What is the Molal BP Elevation Constant (𝐾𝑏) of Water?
What is the Molal FP Depression Constant (𝐾𝑓) of Water?

Answer 7

1. BP Elevation of 1kg of Water at STP: 0.52°C/mol solute
2. FP Depression Constant of Water at STP: 1.86°C/mol solute

Question 8

How do you determine the boiling point of a solution after a solute was added? (5)

Answer 8

1. Determine i
2. Multiply 𝐾𝑓 to i to find Δ𝑇𝑏/mol solute
3. Find number of moles to lower BP by given degrees
4. Find number of moles to lower BP by given degrees in given kg
5. Convert to Grams if Needed

Ex: How many grams of NaCl ro raise the bp of 3.81 L of H2O, by 1.04 Degrees Celsius?

1. i = 1 mol NaCl dissociate into 2 mol of solute ions
2. 0.52°C/mol solute x 2 = 1.04°C/mol NaCl
3. Only 1 mol is needed to lower 1.04, so we can skip this step
4. 1 mol NaCl/1 Kg of H2O = X mol NaCl/3.8 Kg H2O
5. 3.8 mol of NaCl x 58.5g/mol of NaCl = 222.3g

Question 9

How do you determine the freezing point of a solution after a solute was added? (3/5)

Answer 9

1. Determine i
2. Multiply 𝐾𝑓 to i to find Δ𝑇𝑓/mol solute
3. Stoichiometry (solve for 𝑚)

Ex: How many grams of CaCl2 is needed to lower FP of 5.7 kg of H2O at the FP by 11°C?

1. Find how many ions CaCl₂ dissociates to - i = 3 ions
2. How much degrees does 1 mol in 1 kg of CaCl2 lower FP
   (1.86° x 3 = 5.58 °C)
3. 1 mol lowers FP by 5.58, what’s num. of mol to lower 11°C in 1kg?
   (1 mol CaCl₂/5.58 °C FP = X mol CaCl₂/11°C FP)
4. 2 Moles lowers 1 kg by 11°C, what’s num. of moles to decrease 5.7 Kg?
   (2 Moles of CaCl₂/1 Kg H2O = X Moles of CaCl₂/5.7 Kg H2O)
5. Convert Moles to Grams to Answer the Question
   (11.4 Moles CaCl2 = 1,265.4 g of CaCl₂)