3.1.2 Amount of substance Flashcards
What are the diatomic elements?
Hydrogen H2
Nitrogen N2
Oxygen O2
Fluorine F2
Chlorine Cl2
Bromine Br2
Iodine I2
Why are the molecular formulas of phosphorus & sulfur?
P4
S8
What are the molecular formulas of ammonia, methane & hydrogen sulfide?
NH3
CH4
H2S
What are the molecular formulas of hydrochloric acid, sulfuric acid, nitric acid & phosphoric acid?
HCI
H2SO4
HNO3
H3PO4
What is the molecular formula and charge of ammonium?
NH4 (+)
What are the molecular formulas and charges of oxide & sulfide?
O (2-)
S (2-)
What are the molecular formulas and charges of nitrate, sulfate, carbonate, hydrogencarbonate, hydroxide, hydride & phosphate?
NO3 (-)
SO4 (2-)
CO3 (2-)
HCO3 (-)
OH (-)
H (-)
PO4 (3-)
What is relative atomic mass?
Relative atomic mass is the average mass of one atom compared to one twelfth of the mass of one atom of carbon-12.
What is relative molecular mass?
Relative molecular mass is the average mass of a molecule compared to one twelfth of the mass of one atom of carbon-12.
What is the equation for calculating moles for pure solids, liquids, and gases?
moles (mol) = mass (g) / Mr
What is the equation for calculating moles for gases?
PV = nRT
P is pressure (Pa), V is volume (m^3), n is moles, R is the gas constant (8.31), and T is temperature in Kelvin (K).
What is the equation for calculating moles in solutions?
moles (mol dm^-3) = concentration × volume (dm^3)
What is Avogadro’s Constant?
6.022 x 10^23 particles
Avogadro’s Constant is the number of particles in 1 mole of a substance (12 grams of carbon-12).
1 mole of a substance contains 6.022 x 10^23 particles.
The mass of 1 mole of a substance in grams is equal to its molar mass.
What is the formula to calculate the number of particles in a substance?
No of particles = moles of substance (in mol) × Avogadro’s constant
Calculate the number of atoms in a 6.00 g sample of tin metal. (Atomic mass of tin = 118.7)
3.04 x 10^22 atoms
Calculation: moles = 6.00/118.7; number atoms = moles x 6.022 × 10^23.
What is the density formula?
Density (g cm^-3) = mass/volume
Calculate the number of molecules of ethanol in a 0.500 dm³ sample (Density of ethanol = 0.789 g cm^-3, Molar mass of ethanol = 46.0).
5.16 x 10^24 molecules
Calculation: mass = density × volume; moles = mass/Mr; number of molecules = moles x 6.022 × 10^23.
Calculate the number of chloride ions in a 25.0 cm³ solution of magnesium chloride with a concentration of 0.400 mol dm^-3.
1.20 x 10^22 ions
Calculation: moles = concentration × volume; number ions = moles x 6.022 × 10^23.
What is empirical formulae?
The simplest ratio of atoms of each element in the compound.
What is the general method to calculate empirical formula?
Step 1: Divide each mass (or % mass) by the atomic mass of the element
Step 2: For each of the answers from step 1 divide by the smallest one of those numbers to get the ratio.
Calculate the empirical formula for a compound that contains 1.82g of K, 5.93g of I and 2.24g of O.
Step1: Divide each mass by the atomic mass of the element to give moles
K = 1.82 / 39.1 = 0.0465 mol
I = 5.93/126.9 = 0.0467mol
O = 2.24/16 = 0.14mol
Step 2: For each of the answers from step 1 divide by the smallest one of those numbers.
K = 0.0465/0.0465 = 1
I = 0.0467/0.0465 = 1
O =0.14/0.0465 = 3
Empirical formula =KIO3
Deduce the molecular formula for the compound with an empirical formula of C3H6O and an Mr of 116.
C3H6O has an Mr of 58
The empirical formula fits twice into Mr of 116
So the molecular formula is C6H12O2
What is a method to heat a crucible of hydrated salt?
The water of crystallisation in calcium sulfate crystals can be removed as water vapour by heating as shown in the following equation.
CaSO4.xH2O(s) → CaSO4(s) + xH2O(g)
Method:
• Weigh an empty clean dry crucible and lid.
• Add 2g of hydrated calcium sulfate to the crucible and weigh again
• Heat strongly with a Bunsen for a couple of minutes
• Allow to cool
• Weigh the crucible and contents again
• Heat crucible again and reweigh until you reach a constant mass (do this to ensure reaction is complete).
- Large amounts of hydrated calcium sulfate, such as 50g, should not be used in this experiment as the decomposition is likely to be incomplete.
- The lid improves the accuracy of the experiment as it prevents loss of solid from the crucible but should be loose fitting to allow gas to escape.
- Small amounts of the solid, such as 0.100 g, should not be used in this experiment as the percentage uncertainties in weighing will be too high.
3.51 g of hydrated zinc sulfate were heated and 1.97 g of anhydrous zinc sulfate were obtained.
Calculate the value of the integer x in ZnSO4.xH2O.
Calculate the mass of H2O = 3.51 - 1.97 = 1.54g
Calculate moles of ZnSO4 = 1.97/161.5 = 0.0122
Calculate moles of H2O = 1.54/18 = 0.085
Calculate ratio of mole of ZnSO4 to H2O
0.0122/0.0122 = 1
0.085/0.0122 = 7
X = 7
Calculate the concentration of solution made by dissolving 5.00g of Na2CO3 in 250cm3 water.
moles = mass/Mr
= 5/(23.0 x2 + 12 +16 ×3) = 0.0472mol
conc = moles/volume
= 0.0472/0.25 = 0.189 moldm-3
How do you convert concentration measured in moldm-3 into concentration measured in gdm-3.
Multiply by Mr of the substance
conc in gdm-3 = conc in moldm-3 x Mr
(Ions dissociating)
If 9.53g (0.1 mol) of magnesium chloride (MgCl2) is dissolved in 1dm3 of water then the concentration of magnesium chloride solution (MgCl2 aq) would be 0.1mol dm3.
However…
MgCI2(s) +aq → Mg^2+(aq) + 2Cl^-(aq)
However the 0.1 mol magnesium chloride would split up to form 0.1 mol of magnesium ions and 0.2 mol of chloride ions. The concentration of magnesium ions is therefore 0.1 mol dm3 and the concentration of chloride ions is now 0.2 mol dm3.
What is the method to make a standard solution?
What is the method to make a dilution?
Standard solution:
- Weigh the weighing boat containing the required mass of solid on a balance
- Transfer to beaker and reweigh boat
- Record the difference in mass
- Add 100cm3 of distilled water to the beaker. Use a glass rod to stir to help dissolve the solid.
- (Sometimes the substance may not dissolve well in cold water so the beaker and its contents could be heated gently until all the solid had dissolved.)
- Pour solution into a 250cm3 volumetric flask
- Rinse beaker and add washings from the beaker and glass rod to the volumetric flask.
- Make up to the mark with distilled water using a dropping pipette for last few drops.
- Invert flask several times to ensure uniform solution.
Dilution:
- Pipette 25cm’ of original solution into a 250cm3 volumetric
flask
- make up to the mark with distilled water using a dropping pipette for last few drops.
- Invert flask several times to ensure uniform solution.
(Using a volumetric pipette is more accurate than a measuring cylinder because it has a smaller uncertainty)
50 cm3 of water are added to 150 cm3 of a 0.20 moldm-3 NaOH solution. Calculate the concentration of the diluted solution.
150/150 = 1
150/200 = 3/4
0.2 x 3/4 = 0.15 moldm-3
Calculate the volume of water in cm3 that must be added to dilute 5.00cm3 of 1.00 moldm-3 hydrochloric acid so that it has a concentration of 0.050 moldm-3.
5/5 = 1
x/x+5 = 0.05
0.05 = 5/100
x = 95 cm3
In pV=nRT experiments, you use a gas syringe. What are the potential errors with this?
• gas escapes before bung inserted
• syringe sticks
• some gases like carbon dioxide or sulfur dioxide are soluble in water so the true amount of gas is not measured
40 cm3 of oxygen and 60 cm3 of carbon dioxide, each at 298 K and 100 kPa, were placed into an evacuated flask of volume 0.50 dm3. Calculate the pressure of the gas mixture in the flask at 298 K.
20 000 Pa
500 cm3 of methane is combusted at 1atm and 300K. Calculate the volume of oxygen needed to react and calculate the volume of CO2 given off under the same conditions.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
O2 - 1000cm3
CO2 - 500cm3
What are the equations for % Yield and % Atom economy?
percentage yield = actual yield/theoretical yield × 100
percentage atom economy = mass of useful products/mass of all reactants × 100
Why do chemists want high % yield & high % atom economy?
Chemists want a high percentage yield as means there has been an efficient conversion of reactants to products.
Chemists want a high percentage atom economy so that the maximum mass of reactants ends up in the desired product (so minimising the amount of by-product).
What is the method for titrations?
How do you calculate % uncertainty?
% uncertainty = uncertainty/measurement made on apparatus x 100
How can you decrease the apparatus uncertainties?
Either decrease the sensitivity uncertainty by using apparatus with a greater resolution (finer scale divisions) or you can increase the size of the measurement made.
To reduce the % uncertainty in a burette reading it is necessary to make the titre a larger volume. How can this be done?
- Increasing the concentration of the substance in the conical flask.
- Decreasing the concentration of the substance in the burette.
If we calculated an Mr of 203 and the real value is 214, then what is the percentage difference?
214 - 203 = 11
% = 11/214 x100
= 5.41%
If the % uncertainty due to the apparatus < percentage difference between the actual value and the calculated value then there is a discrepancy in the result due to other errors.
If the % uncertainty due to the apparatus > percentage difference between the actual value and the calculated value then there is no discrepancy and all the difference between values can be explained by the sensitivity of the equipment.
