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 = concentration (mol dm^-3) × 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