Calculations involving masses 🏋️‍♀️

• Explain the law of conservation of mass in a closed system.

  The law of conservation of mass is the mass of reactants and products are the same. In a closed system oxygen isn't let in so the products will be the same.
• Explain the law of conservation of an open system?

  In an open system, oxygen is let in which results in a change of mass as a gas is involved.
• What is the equation for conservation of mass?

  Mass of solvent + Mass of solute ----> solution
• What is the calculation for relative formula mass?(Mr)

  Sum of atomic masses (Ar).

  Example:

  magnesium chloride, (MgCl2)

  Ar of Mg = 24

  Ar of Cl2 = 2 x 35.5

  = 71 + 24

  = 95
• What is the calculation for an empirical formula?

  Simplest ratio of atoms.

  Example:

  Find the empirical formula for glucose, C6H12O6.

  divide 6,12 and 6 by common factor which is 6.

  C- 6 ÷ 6 = 1

  H- 12 ÷ 6 = 2

  O- 6÷ 6 = 1

  Answer- CH2O
• What is the difference between empirical formula and molecular formula?

  The empirical formula gives the simplest whole number ratio of atoms in a compound, while the molecular formula gives the actual number of atoms of each element in a compound.
• How do you find the molecular formula of a compound from its empirical formula and relative formula mass?

  Compound X has the empirical formula C2H6N. The Mr of compound X is 88. Find the molecular formula.

  First find Mr of empirical formula:

  (2 x 12) + (6 x 1) + 14

  = 24 + 6 + 14 = 44

  Divide Mr of compound by Mr of empirical formula.

  88 ÷ 44 = 2

  Multiply everything in empirical formula by 2.

  C- 2 x 2 = 4 H- 6 x 2 = 12 N- 1 x 2 = 2

  Answer: C4H12N2
• What is meant by a mole of particles?

  6.022 x 10^23 particles which is the Avogadro Constant.
• How do you calculate the mass of moles in a given mass?

  One mole = mass (g) equal to Ar or Mr.

  Example: NaOHMr = 23 + 16 + 1 = 40g

  1 mole = 40g

  2 moles = 80g

  0.5 moles = 20g
• How do you calculate the number of particles in a given number of moles?

  Avogadro's constant.

  Examples:

  6.02 x 10^23 = 1 mole of carbon in atoms.

  1.204 x 10^24 = 2 moles of oxygen in molecules.

  3.01 x 10^23 = 0.5 moles in ions.
• How do you calculate the number of moles?

  Number of moles = mass of substance / molar mass of substance.

  Example:

  26g of Mg(Ar is 24) is 26 ÷ 24 = 1.08 mol.
• How do you find molar ratios?


  Coefficients.

  Example:

  CH4 + 2O2 ----> CO2 + 2H2O

  For every one mole of CH4, two moles of O2 are used due to the 2 in front of the oxygen.
• Give an example of reacting masses.

  2H2S + 3O2 ----> 2SO2 + 2H2O.

  Calculate the mass of oxygen needed to react with 8.5g of hydrogen sulphide.

  H2S = Mr- 34g Mass- 8.5g Mol- 0.25g

  O2 = Mr- 32g Mass- 12g Mol 0.375g

  Answer- 12g
• How to go from dm3 to cm3?

  1dm3 = 1000cm3
• What is the calculation for concentration?

  mass of solute in g ÷ volume of solution in dm3
• What is the calculation for the volume of solution in dm3?

  mass of solute in g ÷ concentration.
• How do you calculate mass?

  Concentration x volume of solution in dm3.