Quant Chem concepts

What is the FIRST step if a compound is 76.56% C

6.44% H, 17.00% O?, Assume 100 g → 76.56 g C, 6.44 g H, 17.00 g O

You converted % → grams

what is the next step?, Convert grams → moles using n = m/M

You got 76.56 g C

what do you do next?, 76.56 ÷ 12.01 = 6.375 mol C

You got 6.375 mol C

6.376 mol H, 1.063 mol O, what next?, Divide all by smallest (1.063)

You divided and got ~6 : 6 : 1

what do you do next?, Use as subscripts → empirical formula

You got empirical formula C6H6O

what is next step if molar mass is given?, Find empirical molar mass

You got empirical mass = 94.12 g/mol and actual = 188.22 g/mol

what next?, Divide → find multiplier

You got multiplier = 2

what do you do next?, Multiply all subscripts → molecular formula

If 1.12 g NH3 and 0.925 g O2 react

what is FIRST step?, Convert both to moles

You converted 1.12 g NH3 → 0.0657 mol

what next?, Convert O2 to moles

You got 0.0289 mol O2

what do you do next?, Use mole ratios to compare product formed

You found O2 makes less product

what is next?, Identify O2 as limiting reagent

You found limiting reagent

what do you do next?, Use it to calculate final product amount

If Ksp = 1.0×10⁻²² and [Y³⁺] = 0.24 M

what equation do you use?, Ksp = [Y³⁺][OH⁻]³

You wrote Ksp expression

what is next step?, Plug in known values

You got 1.0×10⁻²² = (0.24)(OH⁻)³

what next?, Solve for [OH⁻] (divide then cube root)

You got [OH⁻] = 7.47×10⁻⁸

what do you do next?, Convert to pOH

You got pOH = 7.13

what is next?, Convert to pH

You got pH = 6.87

what do you do next for separation?, Choose slightly LOWER pH

If 48.0 mL of 0.282 M acid reacts with 0.39 M base

what is FIRST step?, Convert to moles using M×V

You got 13.536 mmol acid

what next?, Calculate moles of base added

You got 11.700 mmol base

what do you do next?, Do reaction table (subtract/add)

You got 1.836 mmol acid and 11.700 mmol conjugate base

what next?, Recognize buffer

You identified a buffer

what do you do next?, Use Henderson-Hasselbalch equation

You plug into pH = pKa + log(A⁻/HA)

what next?, Calculate pH

If pH = 5.50

what is the FIRST step?, Convert to [H⁺]

You got [H⁺] = 3.16×10⁻⁶

what do you do next?, Use Ka expression

You plugged into Ka = ([H⁺]²)/(0.061)

what next?, Solve for Ka

You got Ka = 1.64×10⁻¹⁰

what do you do next?, Convert to pKa

You got pKa

what is final step?, Report value

If Ka = 1.4×10⁻⁵ and concentration = 0.25 M

what do you check first?, % ionization (<5%?)

You confirmed approximation works

what do you do next?, Use [H⁺] = √(Ka·C)

You got [H⁺] = 1.87×10⁻³

what do you do next?, Convert to pH

If Ka = 1.10×10⁻²

what is FIRST step?, Convert to Kb

You got Kb = 9.12×10⁻¹³

what do you do next?, Use approximation formula

You used [OH⁻] = √(Kb·C)

what next?, Calculate [OH⁻]

You got [OH⁻] = 6.41×10⁻⁷

what do you do next?, Convert to pOH then pH

If solution contains Na⁺ and CH2ClCOO⁻

what do you recognize?, Weak base present

You identified weak base solution

what is next?, Conclude solution is basic

If solution contains NH₄⁺ and CH2ClCOO⁻

what is FIRST step?, Compare Ka vs Kb

You found Ka > Kb

what do you do next?, Conclude solution is acidic

If Q = 316 and K = 1.2×10³

what is FIRST step?, Compare Q and K

You see Q < K

what do you do next?, Predict shift forward

If ΔG° = -47.2 kJ/mol

what do you do next?, Conclude reaction is spontaneous

If 106 g sugar in 946 g solution

what is FIRST step?, Use mass % formula

You plugged into (106/946)×100

what next?, Calculate percent

If 9.6 g solute gives 0.028 mol and volume is 0.10 L

what do you do next?, Use M = mol/L

You plug in 0.028/0.10

what next?, Calculate molarity

If you see 76.56% C...

what do you recognize?, Empirical formula problem

You recognize empirical formula problem

what do you do next?, Convert % → grams

If you see molar mass given after empirical

what do you do next?, Molecular formula step

If you see Ksp and pH

what do you recognize?, Precipitation problem

If you see weak acid + base added

what do you recognize?, Buffer/titration

If you see small Ka and concentration

what do you recognize?, Approximation