Some Basic Concepts of Chemistry
Domain 1: Some Basic Concepts of Chemistry
This domain tests how confidently you convert a laboratory observation into a number: moles, mass, volume, concentration, and stoichiometric ratio. KSEAB questions often combine significant figures with limiting-reagent reasoning and solution concentration.
Core rules
- The mole is the link: n = m/M = N/N_A = V/22.4 L at STP for an ideal gas. Molar mass of a diatomic gas uses both atoms.
- A balanced equation gives mole ratios, not mass ratios. Identify the limiting reagent by dividing moles by coefficient; the smallest quotient limits.
- Molarity M = mol solute/L solution and changes with temperature; molality m = mol solute/kg solvent does not. Mole fraction and ppm are ratio based.
- Significant figures: addition/subtraction keeps the least decimal places; multiplication/division keeps the least significant figures. Exact integers do not limit.
- Laws of chemical combination and Avogadro's law support the mole-ratio interpretation.
Worked example
5.60 g Fe reacts with 3.65 g HCl: Fe + 2HCl -> FeCl2 + H2. n(Fe)=5.60/56.0=0.100 mol; n(HCl)=3.65/36.5=0.100 mol. HCl needs 0.200 mol for 0.100 mol Fe, so HCl is limiting. n(H2)=0.100/2=0.0500 mol; V=0.0500 x 22.4=1.12 L at STP. Mass is not the limiting test; mole/coefficient is.
Traps
- Using 22.4 L mol^-1 at non-STP; convert T and P or use the ideal gas equation.
- Forgetting diatomic molar masses (H2, O2, Cl2).
- Treating molarity as mol/kg; molality is solvent-based.
- Rounding intermediate values and losing significant-figure marks.
- Assuming the reactant with smaller mass is limiting.