Why isn't the mass number just the neutrons?
An atom with 17 protons and 18 neutrons: is its mass number 35 or 18? And what is its charge? Work through the counting rules one blank at a time.
Try this lessonGet to know chemistry, one idea at a time.
Make sense of atoms, reactions, and formulas. Work through an example, then try one yourself.
No account or card needed. Try an example, then a new question on your own.
Ask about your own questionAtoms & bonding 5
Does sodium share its electron with chlorine?
NaCl is table salt, but is it held together by sharing or by moving an electron? Trace the transfer and the attraction.
Try this lessonWhen two nonmetals meet, do they share or transfer?
Hydrogen cannot pull an electron away from hydrogen, so the two atoms do something smarter: they share. Count the pairs.
Try this lessonIs it sodium chloride or sodium chlorine?
Two words and two numbers stand between you and a correct ionic name or formula. Learn the rule, then balance the charges.
Try this lessonWhich prefix: mono, di, or tri?
Carbon dioxide and carbon monoxide both start with carbon. The prefixes are the only thing telling them apart, so get them right.
Try this lessonPeriodic table 2
Why does sulfur have 6 valence electrons, not 16?
Sulfur has 16 total electrons but only 6 that do chemistry. Learn where those 6 live and how the periodic table reveals them.
Try this lessonWhich is bigger: sodium or chlorine?
Sodium and chlorine sit next door in the same period, but one is much larger. Decide which, then test the down-a-group trend.
Try this lessonReactions 3
Why can't you balance by changing H2O to H4O2?
Water is H2O, not H4O2. Learn how coefficients balance an equation without inventing new molecules.
Try this lessonSynthesis or combustion: how can you tell?
A reaction can be named by its pattern, not its colors or smells. Look at how many substances come in and how many go out.
Try this lessonDoes doubling pressure double the volume?
Double the pressure and the volume halves; double the kelvin temperature and it doubles. Match each law to its direction.
Try this lessonStoichiometry 6
Why is a mole not a weight?
A dozen is 12. A mole is 6.02 x 10^23. Practice dividing particle counts by Avogadro's number, and learn why the mole is not a weight.
Try this lessonIs water's molar mass 17 or 18 g/mol?
The subscript 2 in H2O is easy to miss in a molar mass. Count it once, and every conversion after stays right.
Try this lesson36 g of water: 2 moles or half a mole?
36.0 g of water is 2.00 mol, and 2.00 mol of CO2 is 88.0 g. Practice reading the units to choose divide or multiply.
Try this lessonWhich coefficient becomes your ratio?
Coefficients are ratios. Three moles of O2 in the hydrogen reaction make twice as much water. Learn to read the recipe.
Try this lessonWhich reactant runs out first?
3.0 mol of O2 and 4.0 mol of H2: which runs out first? The ratio decides, not the smaller number.
Try this lessonIs it 60 over 45 or 45 over 60?
If 60 g was expected and 45 g appeared, is that 75% or 133%? The order of the fraction is the whole lesson.
Try this lessonSolutions & acids 3
0.50 mol in 2.0 L: 0.25 M or 4 M?
Half a mole of salt in two liters of solution. Is the answer 0.25 M or 4 M? The definition settles it.
Try this lessonIs pH 3 more acidic than pH 5?
pH 7 is the midway point. Learn which direction means acid strength, and why pH 3 and pH 5 differ by a hundred times.
Try this lessonWhat is the second product of acid plus base?
Acid and base do not vanish. Their ions trade partners and become a salt and water. Fill in the two products.
Try this lessonEnergy & kinetics 1
Is a catalyst used up, or does it just help?
A catalyst helps a reaction finish faster without being used up. Learn the one quantity it lowers, and what it never changes.
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