Quant Chem 2 Final

"A compound is 76.56% C

6.44% H, and 17.00% O by mass. You are asked for the empirical formula. What is your FIRST step?", Assume 100 g of compound and convert % to grams

"You now have 76.56 g C

6.44 g H, and 17.00 g O. What do you do next to move toward empirical formula?", Convert each to moles using n = m/M

"You just calculated 76.56 ÷ 12.01. What does this give you?"

Moles of carbon

"You now have 6.375 mol C

6.376 mol H, and 1.063 mol O. What is the next step?", Divide all values by the smallest mole amount

"You divided by 1.063 and got ~6 : 6 : 1. What do these numbers represent?"

Subscripts in empirical formula

"You got empirical formula C6H6O. The problem now gives molar mass = 188.22 g/mol. What are you trying to find next?"

Molecular formula

"You calculated empirical mass = 94.12 g/mol. What step comes next?"

Divide actual molar mass by empirical mass

"You got 188.22 ÷ 94.12 ≈ 2. What does this number mean?"

It is the multiplier

"You found multiplier = 2. What is the final step?"

Multiply all subscripts

"1.12 g NH3 reacts with 0.925 g O2. You are asked for limiting reagent. What is your FIRST step?"

Convert both masses to moles

"You converted NH3 to moles. What must you do with O2 before comparing anything?"

Convert O2 to moles

"You now have moles of both reactants. What are you comparing to find limiting reagent?"

Amount of product each can form

"You used mole ratios and got two different product amounts. Which one matters?"

The smaller amount

"You found O2 produces less product. What conclusion do you make?"

O2 is the limiting reagent

"You identified the limiting reagent. What do you use it for next?"

Calculate actual product formed

"You have Ksp = 1.0×10⁻²² for Y(OH)₃ and [Y³⁺] = 0.24 M. You are solving for when precipitation begins. What equation do you write?"

Ksp = [Y³⁺][OH⁻]³

"You substituted values into Ksp. What variable are you solving for?"

[OH⁻]

"You isolated (OH⁻)³. What mathematical step is next?"

Take cube root

"You got [OH⁻] = 7.47×10⁻⁸. The question asks for pH. What is next?"

Convert to pOH

"You calculated pOH. What is the next step to reach pH?"

pH = 14 − pOH

"You got pH = 6.87. The problem asks for best separation. What do you adjust?"

Choose slightly lower pH

"You are titrating 48.0 mL of 0.282 M weak acid with 0.39 M strong base. What is your FIRST step?"

Convert volume and molarity to moles

"You found 13.536 mmol acid. What must you calculate for the base?"

Moles of base added

"You now have moles of acid and base. What is the next step?"

Perform reaction stoichiometry

"You subtracted and found leftover acid and conjugate base. What type of system is this?"

Buffer

"You identified a buffer. What equation should you immediately use?"

pH = pKa + log(A⁻/HA)

"You are about to plug into H-H. What values go into the ratio?"

Remaining moles of conjugate base and acid

"You calculated A⁻/HA. What is the final math step?"

Take log and add pKa

"A weak acid solution has pH = 5.50. You are asked to find Ka. What is the FIRST step?"

Convert pH to [H⁺]

"You calculated [H⁺] = 3.16×10⁻⁶. What equation do you use next?"

Ka = [H⁺][A⁻]/[HA]

"You assume x is small. What simplified expression do you use?"

Ka = ([H⁺]²)/(initial concentration)

"You plugged into Ka. What are you solving for?"

Ka

"You found Ka. What does the problem usually want next?"

pKa

"You have Ka. What formula gives pKa?"

pKa = -log Ka

"You are given Ka = 1.4×10⁻⁵ and concentration = 0.25 M. Before solving

what must you check?", Percent ionization

"You estimate % ionization. What formula do you use?"

√(Ka/C) × 100%

"You found % < 5%. What does that allow you to do?"

Use approximation

"What formula replaces the ICE table here?"

[H⁺] = √(Ka·C)

"You calculated [H⁺]. What is the next step?"

Convert to pH

"You are given Ka = 1.10×10⁻² and asked for pH of solution. What is the FIRST relationship you use?"

Ka × Kb = Kw

"You rearranged and found Kb. What formula did you use?"

Kb = Kw / Ka

"You now have Kb and concentration. What shortcut applies?"

[OH⁻] = √(Kb·C)

"You calculated [OH⁻]. What is the next step?"

Convert to pOH

"You got pOH. What is the final step?"

pH = 14 − pOH

"A solution contains Na⁺ and CH2ClCOO⁻ ions. What should you recognize immediately?"

Presence of weak base

"What does a weak base imply about the solution?"

Basic pH

"A solution contains NH₄⁺ and CH2ClCOO⁻. What must you compare?"

Ka vs Kb

"You found Ka > Kb. What does this tell you?"

Solution is acidic

"You calculate Q = 316 and are given K = 1.2×10³. What is your FIRST step?"

Compare Q and K

"You see Q < K. What does that mean for the reaction direction?"

Shifts forward

"You are asked to calculate ΔG°. What formula from the sheet applies?"

ΔG° = -RT lnK

"You calculated ΔG° and got a negative value. What does this mean?"

Reaction is spontaneous

"A solution has 106 g of sugar in 946 g total solution. What formula applies?"

mass % = (solute/solution)×100

"You plug values into mass % formula. What are you solving for?"

Percent composition

"You have 0.028 mol solute in 0.10 L solution. What formula applies?"

M = mol/L

"You plug into M = mol/L. What are you solving for?"

Molarity

"You are reading a problem and see percent composition given. What type of problem is this?"

Empirical formula

"You see empirical formula followed by molar mass. What is the next concept?"

Molecular formula

"You see Ksp and pH mentioned together. What does that indicate?"

Precipitation problem

"You see weak acid and added strong base. What situation is this?"

Buffer/titration

"You see small Ka and concentration given. What method applies?"

Approximation

"You see pH given directly in a problem. What are you likely solving for?"

Ka or Kb

"You are halfway through a buffer problem but haven't done a reaction step. What did you miss?"

Stoichiometry comes first

"You see both HA and A⁻ present after reaction. What does that confirm?"

Buffer system

"You calculate A⁻/HA > 1. What does that tell you about pH?"

pH > pKa

"You are writing a Ksp expression. What must be excluded automatically?"

Solid compounds

"You are writing Ka or Kb. What general format do you follow?"

Products over reactants

"You are given pH. What formula connects it to concentration?"

[H⁺] = 10⁻pH

"You are given pOH. What formula connects it to concentration?"

[OH⁻] = 10⁻pOH

"You need to convert between pH and pOH. What relationship do you use?"

pH + pOH = 14

"You see a dilution problem with initial and final concentrations. What formula applies?"

C₁V₁ = C₂V₂

"You see molarity and volume in a problem. What formula connects them?"

n = M×V