Empirical Formula Calculation and Stoichiometry in Chemistry

"A compound used as a solvent is found to contain 66.7% carbon and 33.3% hydrogen by mass

and you are asked to determine its empirical formula. Before performing any mole calculations, what assumption must you make to convert percentages into usable quantities?", Assume 100 g → 66.7 g C, 33.3 g H

"After converting percentages into grams

how do you convert each element into moles and what are the results?", C = 66.7/12.01 = 5.55 mol, H = 33.3/1.01 = 32.97 mol

"After calculating moles of each element

what step must you take to determine the simplest whole-number ratio?", Divide by smallest (5.55): 1 : 5.94 ≈ 1 : 6

"What is the empirical formula?"

CH6

"A compound contains 28.0% nitrogen and 72.0% oxygen by mass. What is the first step in determining the empirical formula?"

Assume 100 g → 28.0 g N, 72.0 g O

"Convert each element to moles."

N = 28.0/14.01 = 2.00 mol, O = 72.0/16.00 = 4.50 mol

"What is the mole ratio?"

Divide by 2.00 → 1 : 2.25 → multiply by 4 → 4 : 9

"What is empirical formula?"

N4O9

"8.00 g of aluminum reacts with 16.0 g of oxygen to form aluminum oxide. You are asked to determine the limiting reagent. What is your first step?"

Convert to moles: Al = 8.00/26.98 = 0.296 mol, O2 = 16.0/32.00 = 0.500 mol

"Using the balanced equation 4Al + 3O2 → 2Al2O3

what must you compare to find the limiting reagent?", Required ratio: 4 Al : 3 O2 → need 0.222 mol O2 for 0.296 Al, but have 0.500 → O2 excess

"What is the limiting reagent?"

Al

"12.0 g of Ca reacts with excess Cl2 to form CaCl2. How many grams of CaCl2 are produced? What is the first step?"

Convert Ca to moles: 12.0/40.08 = 0.299 mol

"What is mole ratio between Ca and CaCl2?"

1 : 1 → 0.299 mol CaCl2

"What is mass of CaCl2 formed?"

0.299 × 110.98 = 33.2 g

"A reaction produces 25.0 g of product but the theoretical yield is 40.0 g. What is the percent yield?"

(25.0/40.0)×100 = 62.5%

"Ksp = 5.0×10⁻¹³ for AgCl. If [Ag⁺] = 0.0020 M

what equation do you write?", Ksp = [Ag⁺][Cl⁻]

"Solve for [Cl⁻]."

[Cl⁻] = (5.0×10⁻¹³)/(0.0020) = 2.5×10⁻¹⁰

"A solution contains 0.010 M Ag⁺ and 0.020 M Cl⁻. If Ksp for AgCl is 1.8×10⁻¹⁰

will a precipitate form? What is the first step?", Calculate Q = (0.010)(0.020) = 2.0×10⁻⁴

"How do you compare Q and Ksp and what does it mean?"

Q > Ksp → precipitate forms

"If Q = 0.005 and K = 2.0

what does this tell you about the reaction direction?", Q < K → reaction shifts forward

"If Q = 50 and K = 10

what happens?", Q > K → shifts left

"A solution is prepared by dissolving 10.0 g of NaCl in enough water to make 500.0 mL of solution. What is the molarity?"

moles = 10.0/58.44 = 0.171 mol → M = 0.171/0.500 = 0.342 M

"A solution contains 0.150 mol of solute in 750 mL of solution. What is molarity?"

M = 0.150/0.750 = 0.200 M

"A stock solution of 2.00 M is diluted to 0.500 M. If 25.0 mL is used initially

what is final volume?", 2.00×25.0 = 0.500×V2 → V2 = 100 mL

"A solution contains 18.0 g solute in 90.0 g solution. What is mass percent?"

(18.0/90.0)×100 = 20.0%

"During a reaction

you calculate moles of reactants but are unsure what to do next. If the problem asks for product formed, what is your next step?", Use mole ratio from balanced equation

"If a problem gives grams of reactants and asks for grams of product

what is the correct sequence of steps?", grams → moles → mole ratio → moles product → grams product

"If you finish a stoichiometry problem and still have leftover reactant

what does that indicate?", Excess reagent

"If two reactants are given and one produces less product

what does that automatically mean?", That reactant is limiting

"If you are asked which compound precipitates first

what property do you compare?", Ksp values (smaller Ksp precipitates first)

"If writing a Ksp expression for BaSO4

what is the correct form?", Ksp = [Ba²⁺][SO₄²⁻]

"If writing a Ksp expression for Fe(OH)3

what is the correct form?", Ksp = [Fe³⁺][OH⁻]³

"If a solid is present in an equilibrium expression

should it be included?", No

"If a reaction is at equilibrium and more reactant is added

what happens?", Shifts toward products

"If a reaction is at equilibrium and product is removed

what happens?", Shifts forward

"If temperature increases for an endothermic reaction

what happens?", Shifts toward products

"If temperature increases for an exothermic reaction

what happens?", Shifts toward reactants

"If you are solving a dilution problem

what formula must you use?", C1V1 = C2V2

"If you are converting molarity and volume to moles

what formula do you use?", n = M×V

"If you calculate Q and it equals K

what does that mean?", System is at equilibrium

"If you see "before equilibrium" in a problem

what table do you use?", ICE table

"If you see "after reaction" in stoichiometry

what do you do?", Subtract reactants, add products

"If you are given grams and need molarity

what must you do first?", Convert grams → moles

"If volume is given in mL but molarity requires L

what must you do?", Convert mL → L