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Chemical Formulae

Chemical Formulae. Learning Objectives: Calculate the formula mass of elements and compounds Explain how the amount of a substance can be measured. Mass number.

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Chemical Formulae

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  1. Chemical Formulae Learning Objectives: Calculate the formula mass of elements and compounds Explain how the amount of a substance can be measured

  2. Mass number Electrons have a mass of almost zero, which means that the mass of each atom results almost entirely from the number of protons and neutrons in the nucleus. The sum of the protons and neutrons in an atom’s nucleus is the mass number. It is the larger of the two numbers shown in most periodic tables.

  3. Relative atomic mass is normally given the abbreviation Ar. We can also calculate the mass of a compound. This is called therelative formula mass, Mr To do this, we need to find the Ar of all the elements in the compound. We can do this by looking at the periodic table or it will be given to you.

  4. Ar: C =12,H = 1,Ca = 40,N =14, 0 = 16 Methane CH4 Relative formula mass = (1 x12) + (4x1) =16 Calcium nitrate, Ca (NO3)2 Relative formula mass = (1x40) + (2x14) + (6x16) =164 Examples

  5. BINGO!

  6. Relative atomic mass – examples 12 atoms x 1= 1 atom x 12 C H H H H H H H H H H H H H H H H H H H H H H H H 1 atom x 24= 2 atoms x 12 Mg Mg C C The lightest atom is hydrogen. It has 1⁄12 the mass of carbon and so has a RAM of 1. Magnesium is twice as heavy as carbon. It has a RAM of 24.

  7. How many atoms are there in 12 g of carbon (one mole)? 602 257 000 000 000 000 000 000 or 6.022 x 1023 This is the number of atoms in 12 g of carbon or one mole of particles. It is called the Avogadro constant.

  8. Moles in Equations Sodium + Chlorine Sodium Chloride Na + Cl NaCl Therefore, the equation is really telling us that we need to mix 6.022 1023 sodium atoms with 6.022 1023 chlorine atoms. to avoid using numbers with such absurd exponents as 1023, chemists decided to name the number of particles present in the relative atomic mass of an element when expressed in grams as the mole: One mole = 6.022 1023 atoms

  9. How to calculate the number of moles in grams (g) Find out the number of moles of a substance in a equation

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