Grade 11 Chemistry

If apples cost 2 dollars each and you spent 18 dollars, you bought 9 apples: you divided the total by the price of one. Molar mass is the price of one mole, in grams.

Solve it for the mass and you get m = nM — the recipe question, how much do I weigh out?

Read the subscript aloud as “N sub A”; the A is for Avogadro, who never measured it and has it named after him anyway, which is science being generous.

Weigh the X atoms in ONE formula unit, divide by what that whole formula unit weighs, and multiply by a hundred. Notice it needs no sample at all — a speck of water and a lake of water are both 11.1 % hydrogen by mass.

Here is the map of this whole chapter, and it is worth memorising as a picture: grams ⇄ moles ⇄ particles.

Every synthesis has two endings. On paper, stoichiometry works out a maximum — the theoretical yield, a prediction computed before the flask is even dirty.

Keep the flask's number on TOP. Both entries are masses of the same product, so the grams cancel completely: a percent yield is a pure number, and it travels without units, like the coefficients of friction do in physics.

The yield formula runs backwards as happily as forwards. Know the promise and the yield?

At this rung the words in the question already say which way the heat flows, so compute with magnitudes and let “released” or “absorbed” speak the sign.

Grams → moles → kilojoules: two small formulas, one chain, and the mole count is the coupling between them.

Before any calculator, run the one-digit version: round every value to a single digit, multiply or divide in your head, and count the zeros. 3.9 of 5.2 grams?

Chemists nickname it molarity, and a 2 mol/L bottle gets called “two molar” — written 2 M for short. The slash is the tell. mol/L divides a mole count by a volume — that is molarity.

Twice the solute in the same flask, twice the concentration; the same solute in twice the water, half. The unit mol/L says exactly what the formula does — moles upstairs, litres downstairs.

Solved for the moles it reads n = C V — how much solute a measured pour actually carries.

That is the whole trap of this lesson, so hear it twice: the solute is PART of the solution it sits in. The denominator is solute plus solvent, added before anything is divided.

Add water to a solution and one thing refuses to change: the moles of solute. Water brings volume and nothing else — every particle that was in the flask is still in the flask.

Keep the subscripts honest — 1 is the strong start, 2 is the watery finish — and every rearrangement is one division away.

Nothing on a stockroom shelf was born a solution.

Every gas problem in this chapter turns the same four dials: pressure P in kilopascals (the gas drumming on its container), volume V in litres (the room it fills), temperature T (how hard the particles jiggle), and amount n in moles (the…

Trap some gas, hold its temperature steady, and squeeze: the volume drops, the pressure climbs, and their PRODUCT never moves.

One question sorts every problem: can it swell? Free to swell is Charles; locked rigid is Gay-Lussac. And here the kelvin doctrine collects.

Real gases rarely change one thing at a time. A balloon that climbs meets thinner air AND colder air together.

The ratio laws compare a gas to ITSELF, before and after. The ideal gas law needs no before: PV = nRT — P V equals n R T — pins down a single state, because it knows something the ratios never asked: how MUCH gas is there.

PV = nRT is one fact wearing four outfits: solve it for P, V, n or T and it answers four different exam questions. Solved for n it is a gas COUNTER — the stockroom's way of auditing a cylinder without opening it.

Helium, oxygen, carbon dioxide — same rent, because ideal gas particles are all equally standoffish regardless of their size.

Gases have weight — a litre of the air around you carries about 1.2 g.

A litre of water weighs a kilogram — a thousand grams, a million milligrams. So one milligram dissolved in one litre is one part in a million: 1 mg/L = 1 ppm — read aloud: one milligram per litre IS one part per million.

Hardness is dissolved calcium and magnesium — the pair behind kettle scale and stubborn soap scum. The trade reports both in a single currency, mg/L as CaCO₃: as if every hardness ion were chalk.

The grain is 64.8 mg — the mass of a barley seed, one of the oldest units still drawing a paycheque. Water-softener dials across North America speak grains of hardness per US gallon, while every lab report speaks mg/L as CaCO₃.

Calcium carbonate's is the trade's favourite number: M = 100.09 g/mol, z = 2, so EW = 50.04 — almost a round fifty. That near-roundness is exactly WHY chalk got elected the yardstick.

Pure water barely conducts; dissolved ions carry the current.

Feed chlorine to raw water and the water eats first: iron, manganese, organics and ammonia consume their share before any protection is left over.

MGD is million US gallons per day, the flow unit of the trade, and 8.34 is what one gallon of water weighs in pounds.

What the compression buys is a ruler that runs about 0 to 14: 7 is neutral (pure water, room temperature), below 7 is acidic — the lower, the fiercer — and above 7 is basic.

The resident trap is that minus sign. Forget it and pH 3 becomes −3, which would out-acid anything in a school stockroom — if your pH comes out negative on this paper, the sign slipped; go back one line.

Every machine in this room runs backwards.

In any water solution at room temperature the two ends are chained together: pH + pOH = 14 — read aloud: pH plus pOH equals fourteen.

Because pH is a logarithm, its steps do not add — they multiply.

A titration measures a concentration you cannot see by reacting it away with one you know.