Structure 1.4 Counting particles by mass: the mole

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Structure 1.4.1 Structure 1.4.2 Structure 1.4.3 Structure 1.4.4 Structure 1.4.5 Structure 1.4.6

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25 Terms

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Avogadro’s constant

6.02 × 10²³

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Calculating the amount of substance (N)

N= n (no.of moles) x Na (avagadro’s constant)

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Define relative molecular mass (Mr)

The ratio of the mass of a molecule to one-twelfth of the mass of a carbon-12 atom.

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Calculating Mr

Add all the Ar’s for all the atoms in that molecules

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Define molar mass (M) (g/mol)

mass of 1 mol of that substance
[= to relative atomic mass]

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relationship between no.of moles, mass and molar mass

n= m/M

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Calculating the number of atoms of an element in a compound

  1. Identify the no.of atoms of that element present in that compound

  2. multiply the n of that element to the n of the compound (n=m/M)

  3. Substitute the no.of moles into N=n/M

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Molecular formula

shows the actual no.of atoms of each element in the molecule of that substance

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Empirical formula

shows the simplest ratio of atoms of different elements that are present in the substance

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Calculating the % composition of a substance

  1. Find the mass of each of the elements present [m=n x M]

  2. Find the mass of the entire compound

  3. Divide the mass of the element with the mass of the compound and divide it by 100

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Calculating the empirical formula

  1. mass of the elements

  2. find no.of moles of the elements

  3. divide n by the smallest value of n

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Calculating the empirical formula of the reactants from the product

  1. mass of the elements

  2. n of the elements

  3. the n of the reactant= the no.of atoms of the reactant x the n of the compound

  4. the mole ratio

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Calculating empirical formula from the percentage composition of an element

  1. Mass of each element

  2. Find the mass of the other element [100-x]

  3. Find n of both the elements

  4. divide the values by the smallest number

  5. Mole ratio of the elements

  6. multiply by a factor for whole number

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Determining molecular formula of a compound using the empirical formula using the molar mass (with given molar mass)

  1. use the empirical formula values to find the molar mass of the empirical formula [M x no.of atoms of that element]

  2. Calculate the molar mass of the empirical formula

  3. Divide the given molar mass by a number to obtain the calculated molar mass

  4. Multiply the empirical formula by the remainder of the division

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Calculating concentration (mol/dm³)

concentration (c ) (solute)= no.of moles (n) (solute)/ volume (V) (solution)

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relationship between mass concentration, molar concentration and molar mass

Molar concentration (P)= mass concentration (c ) x molar mass (M)

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Calculating concentration of a solute in a new solution after transferring a sample of it from a standard solution (mass of solute, volume of volumetric flask (standard solution), volume of pipette, volume of volumetric flask (new solution given)

  1. calculate the concentration of solute in standard solution [c=n/v]

  2. calculate the n of the solute in the sample [c=n/v]

  3. n(sample)= n(new solution)

  4. calculate the concentration in new solution

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Calculating concentration of a solute in a new solution

required:

  1. original concentration of the solute

  2. volume of the original solution

  3. volume of the new solution
    [C1 x V1= C2 x V2]

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What is spectrophotometry and what is it used for?

analytical technique based on measurement of the intensity of visible, UV and near-IR radiation.

Used for determining concentrations of colored substances in solutions

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Absorbance

the amount of light absorbed by the sample

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Caliberation curve

Absorbance against concentrations

used for determining the unknown concentration of the colored substance in a solution

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Ideal Caliberation curves

  • Liner

  • Pass through the origin

  • 45 degree tilt

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Avogadro’s law

Equal volumes of any 2 gases at the same temperature and pressure contain equal numbers of molecules

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Relationship between no.of moles and volume

(N1/N2)=(V1/V2)

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Calculating volume of gases in combustion reactions (Volume of 1 substance given)

  1. Identify the ratio of the stoichiometric coefficients

  2. multiply the ratio with the volume given to find the volume of the other gas (eg. 2:3 = 3/2 x volume)