Calculations Flashcards
Calculating mass
mass = molar mass x moles m = M x n
Calculating moles
moles = mass/molar mass n = m/M
Units for mass, moles and molar mass
mass = g
moles = mol
molar mass = gmol^-1 or g/mol
RAM
Relative Atomic Mass
The mass of an atom is so small, it is more convenient to know their masses compared to each other. Carbon is taken as the standard atom and has a relative atomic mass.
Avogadro’s number
6.022 x 10^23 is Avogadro’s number (N(subscript)A).
1 mole of any substance = 6.022 x 10^23 units of that substance.
Moles
A mole is the amount of any substance that contains the same number of units as the number of atoms in exactly 12 grams of carbon-12.
Molar Mass
Molar mass is the mass in grams of one mole of a substance.
N (particles)
n x N(subscript)A
Avogadro’s Number
6.02 x 10^23 = N(subscript)A
Mass Percentage equation
Percentage by mass = molar mass of element/molar mass of compound x 100
Mass percentage example
What is the mass percentage of Sulfur in Sulfur dioxide?
Molar mass (SO2) = 64.1 g/mol Molar mass (S) = 32.1
Percentage by mass = 32.1/64.1 x 100 = 50.1%
Purity percentage
Mass of pure substance/ mass of impure substance x 100
Definition of empirical formula
Shows the simplest whole number ratio of elements present in a compound.
Definition of molecular formula
Shows the actual number of atoms in a molecule.
How to work out empirical formula (e.g. 10g of C and 1.6g of H)
- rewrite mass of each element (same with percentage)
- divide mass of each element by RAM
- divide each answer by smallest number
- whole numbers are now the ratio
e.g.
C = 10g H = 1.6g
10/12 = 0.83 1.6/1 = 1.6
0.83/0.83 1.6/0.83
= 1 = 2
–> CH2
How to work out molecular formula (e.g. EF: CH2 and molar mass = 84g/mol)
- find the molar mass of the empirical formula
- divide the molecular formula molar mass by the empirical formula molar mass
- use this number to multiply the empirical formula ratio
e.g. CH2 = 12 + 1 + 1 = 14g/mol 84/14 = 6 1:2 x 6 = 6:12 C6H12
Molar Volume of a gas
V = Vm x n
value of V
Volume in L
volume of a gas
V = Vm x n
Molar volume value
Vm = 24.8
Definition of molar volume of gas
“1 mole of any gas at room temperature (25*C) and room pressure (1 atm) occupies a volume of 24.8L”
Quantity unit conversions
1000 ml = 1L = 1000cm3
1cm3 = 1 mL
24800 cm3 = 24800 mL = 24.8 L
Theoretical Yield
Calculated mass (g) or volume (L).
Actual Yield
Mass (g) or volume (L) actually produced in an experiment.
Percent Yield
How much of the theoretical yield was actually produced.
Molar concentration
moles per litre
mol/L, molL^-1, M = unit of concentration
molar concentration formula
C = n/V
molar concentration = moles of solute/volume of solution
n = C x V
moles of solute = volume of solution x molar concentration
Titration
A way to measure volumes precisely.
Method:
- Place burette on the floor with the tap closed. Using a funnel carefully fill the burette with NaOH.
- Measure out 20mL of HCl from a beaker using a glass pipette along with a pipette head and deliver this to a clean conical flask.
- Add 3 or 4 drops of phenolphthalein indicator to the conical flask. This should stay colorless.
- Place a white tile at the base of the burette stand and place the conical flask on the tile.
- Record the initial reading from the burette to 2 decimal places.
- Slowly deliver the NaOH into conical flask, drop by drop.
- When a single drop causes the color to permanently remain, stop and record the final reading.
- Repeat steps until at least 3 concordant titres are obtained.
Features in method to improve accuracy
- Potential contaminants removed
- Flask is clean to reduce contamination.
- Need enough indicator to see a color change, but not so much as to change volume of solution substantially.
- White tile allows color change to be seen more clearly - we want persistent hint of pink.
- Repeat to get concordant results for reliability and to calculate the mean average for greater accuracy.
Standard Solution
Accurately known concentration.
Aliquot
Indicator
Solution added to give visual indication of the end point.
Equivalence point
Marks the stage at which the reaction is complete, when neither reagent is in excess.
End Point
The point at which the acid and alkali have reacted completely. The indicator color change marks the end point.
