--- module: 105-01 language: en chapter: 105 title: "Quantitative Chemistry and Acid–Base Foundations" module_title: "Amount, concentration, stoichiometry, and experimental yield" source_sha256: c510b70117c21413012902842174970a83c31f531535209875c1bb0bb224a777 --- # Amount, concentration, stoichiometry, yield ## Three separate quantities ### How much substance exists ### How much space its solution occupies ### How much substance reacts ### Concentration can fall while amount is unchanged ### Balanced equation limits conversion, not speed or recovery ## From mass to amount ### Mole counts specified entities #### Exactly 6.02214076 x 10^23 entities ### Calcium chloride: one formula unit mole #### One mole calcium, two moles chlorine atoms #### Dissolves to calcium ions and chloride ions #### Not three moles of calcium chloride ### Molar mass converts grams to moles #### 3.60 g at 120 g/mol is 30.0 mmol #### Millimole is a thousandth of a mole ### Equal masses usually differ in entity number ### Empirical versus molecular formula #### Empirical: simplest whole-number ratio #### Molecular: actual composition of a molecule #### CH2O at 30 and measured 180 gives multiplier six #### Ratio alone cannot identify the compound ## Concentration and dilution ### Molarity per litre of final solution #### Not per litre of added solvent ### Aliquot diluted to a final volume #### 15.0 mL transfers 3.00 mmol #### Made up to 100 mL gives 0.0300 M #### Adding 100 mL is a different operation ### C1V1 = C2V2 conserves the chosen solute #### Assumes no loss, generation, or consumption #### Reacting mixtures need a reaction calculation ### Mixing same nonreacting solute #### Add amounts, divide by combined volume #### Plain average only for equal volumes ### Successive dilution factors multiply ## Concentration units ### Mass percent: solute mass over solution mass ### Mass per volume percent: g per 100 mL #### 2% w/v is 20 g/L or 20 mg/mL ### Molality: moles per kilogram of solvent #### Linking to volume needs density and composition ### Density links mass and volume #### 10.0% by mass at 1.20 g/mL #### 100 mL weighs 120 g, holds 12.0 g solute #### At 60.0 g/mol gives 2.00 M #### Needs both density and molar mass ## Balancing before calculating ### Conserve elements and ionic charge ### Change coefficients, never subscripts #### Subscripts identify the substance #### Coefficients specify how much participates ### Ethanol combustion ratio 1:3:2:3 #### Coefficients are mole ratios, not gram ratios ## Limiting reactant and mass check ### 0.080 mol ethanol with 0.180 mol oxygen #### Oxygen supports only 0.060 mol, so it limits ### 0.120 mol CO2, 0.180 water, 0.020 ethanol left ### Raw mole comparison misleads when ratio is not 1:1 ### Mass check: 9.44 g in, 9.44 g out #### Catches an omitted excess reactant #### Open vessel may lose gas without violating conservation ## Predicted conversion versus recovery ### Theoretical yield from reaction and limiting input ### Actual yield reflects further factors #### Conversion and competing reactions #### Collection losses and measurement quality ### 4.22 g of 5.28 g is about 79.9% #### Percentage does not identify the cause ### Yield above maximum signals a problem #### Wet product, impurity, wrong concentration or identity #### Not evidence that atoms were created ### Extra excess reactant does not raise the ceiling #### May still alter rate or equilibrium ## Precision and accuracy ### Close agreement can share a systematic bias ### Replication characterises variability only ### Uncertainty in both numerator and denominator ## Species, particles, and reporting ### Analytical concentration versus species concentrations ### 0.050 M CaCl2 gives 0.050 M Ca and 0.100 M Cl #### Ideal 0.150 osmol/L of particles #### Approximation, not measured osmolarity ### Charge equivalents: two per mole of calcium ### Exact coefficients do not limit significant figures ### Keep extra digits, round the final result ### Final checks before accepting an answer #### Does the unit name the requested quantity #### Did dilution change amount or concentration #### Theoretical maximum or observation