Chemical Calculations Notes 1

The relative formula mass of a compound is calculated by adding up the atomic masses of all the atoms in the compound. In lithium oxide (Li2O), there are 2 lithium atoms and 1 oxygen atom. The atomic mass of lithium is approximately 6.94 atomic mass units (amu) and the atomic mass of oxygen is approximately 16.00 amu. Adding up the atomic masses of the atoms in Li2O, we get: (2 x 6.94) + 16.00 = 30. Therefore, the relative formula mass of lithium oxide is 30 amu.

The relative molecular mass of oxygen is 32 because in oxygen gas (O2), there are two oxygen atoms. The atomic mass of oxygen is 16, so when you multiply it by 2 (because there are two oxygen atoms), you get a relative molecular mass of 32.

Oxygen and bromine are both elements with different atomic masses. The question is asking how many oxygen atoms would have the same mass as one bromine atom. Since the atomic mass of oxygen is smaller than that of bromine, it would take multiple oxygen atoms to have the same mass as one bromine atom. The answer is 5 because the atomic mass of bromine is approximately 80 grams per mole, while the atomic mass of oxygen is approximately 16 grams per mole. Therefore, it would take 5 oxygen atoms with a total mass of 80 grams to have the same mass as one bromine atom.

The relative molecular mass of dinitrogen pentoxide (N2O5) is 108. This is calculated by adding up the atomic masses of each atom in the molecule. Nitrogen (N) has an atomic mass of 14, and oxygen (O) has an atomic mass of 16. Since there are two nitrogen atoms and five oxygen atoms in dinitrogen pentoxide, the total molecular mass is (2 x 14) + (5 x 16) = 108.

The relative formula mass of copper sulphate pentahydrate CuSO4.5H2O is 250. This is calculated by adding up the atomic masses of each element in the compound. Copper (Cu) has an atomic mass of 63.5, Sulphur (S) has an atomic mass of 32, Oxygen (O) has an atomic mass of 16, and Hydrogen (H) has an atomic mass of 1. Multiplying the atomic mass of each element by the number of atoms present in the compound and adding them together gives a total of 250.

One molecule of ethane has 2 carbon atoms.
One mole of ethane would have 6 x 10^23 ethane molecules.
Hence the number of carbon atoms present would be
2 x 6 x 10^23 = 1.20 x 10^24.

[Remember your 3 significant figures]

The mass of 0.500 mol of water is 9.00 g. This is because the molar mass of water is approximately 18.00 g/mol, which means that one mole of water weighs 18.00 grams. Therefore, 0.500 mol of water would weigh half of that, which is 9.00 grams.

Remember to leave your answer to 3 significant figures.
Unit for mole as a "unit" is mol (not mole)!!

Mr(O2) = 32
Hence n(O2) = 64/32 = 2.00 mol

The answer is 4.00 x 10^23 mol because there are 2 oxygen atoms in each molecule of oxygen gas. Therefore, if there are 2.00 x 10^23 molecules of oxygen gas, there will be double the number of oxygen atoms, which is 4.00 x 10^23 mol.