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IB Stoichiometry 1.2
the mole concept and avogadro's constant
| Question | Answer |
|---|---|
| Avogadro's constant (L) = | 6.02 x 10^23 mol-1 |
| What does avogadro's constant mean... | It is the number of atoms or molecules or ions in 1 mole of a substance. |
| Calculating Number of moles (n) | number of particles (N) / 6.02 x 10^23 (L) |
| Water of crystallization... when heated... | is when compounds contain water molecules chemically bonded into the structure of the crystals. It is when compounds are hydrated. When heated the water is given off and compound is dehydrated. |
| Relative atomic mass (Ar) | the mass of an average atom of the element, relative to an atom of carbon-12 isotope (mass of 12g) |
| Relative molecular mass (Mr) | mass of a molecule |
| Ar and Mr do not... | have units |
| Atomic mass of a substance is... | the mass of one mole of that substance (g.mol-1) |
| Molecular mass of a substance is... | the mass of one mole of the molecules (sum of atomic masses of component elements) |
| Formula mass of an IONIC substance is... | the sum of the atomic masses of the atoms in the formula (including water of crystallisation if ionic substance is hydrated) |
| Number of moles (n) = | mass in g (m) / molar mass in g mol-1 (M) |
| Percentage composition | 1. find proportion by mass of each element 2. convert it to a percentage i.e. (mass of desired element / molar mass) x 100 |
| Empirical formula (EXPERIMENTAL) | simplest whole number ratio of the atoms of each element in the compound |
| How to find the empirical formula | 1. find moles of each element present (n=m/M) 2. divide all moles from smallest value 3. get the simplest whole number ratio 4. can round up 5. in some cases multiplying by a small integer will be necessary to get whole number ratio |
| Finding Percentage composition of ORGANIC COMPOUNDS | 1. Burn known mass of compound in excess oxygen 2. Moles of CO2 (C) formed (n=m/M) 3. Moles of H2O formed (x2 for moles of H) 5. Mass of C+H 6. Mass of O = initial mass - mass of C+H 7. Find mole ratio to get the formula! |
| Molecular Formula (THEORETICAL) | 1. Find the factor ( theoretical molar mass/empirical molar mass) 2. Multiply empirical formula by this factor |
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