4 mistakes learners make with chemical calculations and moles, each one named and explained.
A mole is a fixed number of particles, so the same mass of two different substances is not the same number of particles.
You say 10 g of hydrogen and 10 g of oxygen contain the same number of molecules.
Mass is the quantity you can see on the balance, so it stands in for amount.
Question. Which contains more molecules, 16 g of methane or 16 g of oxygen?
A common answer. they contain the same number
The answer. the methane, because its molecules are lighter
Why. Lighter molecules means more of them fit into the same mass.
Convert to moles before comparing amounts. Mass alone does not count particles.
You say the mass cannot change because nothing was added or taken away, even though the container is open.
Mass is conserved is the rule you have learned, and a gas leaving is invisible.
Question. Marble chips react with acid in an open flask. What happens to the mass?
A common answer. it stays the same, because mass is conserved
The answer. it falls, because carbon dioxide leaves the flask
Why. Mass is conserved overall, but the gas carrying it has left the flask you are weighing.
Ask whether anything can leave. Conservation applies to everything, not to the flask.
You count the atoms in 2H2O as 2 hydrogens and 1 oxygen, or as 2 of each.
Both numbers are written near the letters, and only one of them is read aloud in class.
Question. How many hydrogen atoms are in 2H2O?
A common answer. 2
The answer. 4
Why. The 2 in front multiplies the whole formula, and each H2O holds 2 hydrogens.
Multiply the two together for the atom count, every time.
You change a small number inside a formula, turning H2O into H2O2, so the oxygens come out even.
The subscripts are the numbers in front of you when the atoms do not match, so they look like the ones to adjust.
Question. Balance: H2 + O2 gives H2O
A common answer. H2 + O2 gives H2O2
The answer. 2H2 + O2 gives 2H2O
Why. Changing a subscript makes a different substance. Only the big numbers in front may change.
Never touch a subscript. Balance with the big numbers only.
WAJD spots these patterns in your child's answers and names the one behind their wrong answers, instead of just marking them wrong.