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# AQA A1 Calculations

### AQA calculations

What are isotopes Atoms of the same element that have the same number of PROTONS (atomic) but different number of NEUTRONS (mass)
What is the standard for Relative Mass, to which all other masses of atoms are compared to? Carbon-12
What is the relative Isotopic Mass? the mass of an atom of an isotope, compared with 1/12th of the mass of an atom of Carbon-12
What is the Relative Atomic Mass (Ar)? the mean mass of an atom weighed according to the abundance of the isotopes (and compared with 1/12th of the mass of an atom of Carbon-12)
What is the Relative Molecular Mass? the mean mass of a molecule weighed according to the abundance of the isotopes of each element(and compared with 1/12th of the mass of an atom of Carbon-12)
What is the Relative Formula Mass? The same as the RMM, but applied to giant structure (that do not exist as molecules!)
What is the RAM of Boron, when there is 19.77% of Boron-10, 80.23% of Boron-11? (give to 4sf) 10.80
Work out the RAM of Chlorine knowing that there are only two isotopes Cl-35 and Cl-37 and that there is 3 times more Cl-35 than Cl-37 Set up the equation: RAM= (sum of abundancexmass) / total abundance; here there are 3 parts Cl-35 to 1 part Cl-37 so total abundance is 4; RAM=(35*3+37*1)/4=35.5
The RAM of Silicon is 28.11 a.u. . what does it mean in terms of abundance of isotopes? There is more atoms of isotope-28 than there is of any other isotopes.
The RAM of Rubidium is 85.47; there are only two isotopes: Rb-85 and Rb-87 ;work out the abundance of Rb-85 Set up the equation: RAM= (sum of abundancexmass) / total abundance =85.47=[85*x+87*(100-x)]/100; then solve the equation; x=76.5
What does Avogadro's constant represent? The number of atoms in a mole of Carbon-12
What is the value of Avogadro's number? 6.02x10^23 atoms per mole
What is the Molar Mass? The mass of a mole of a substance.
What is the unit of Molar Mass? g.mol-1
How do you work out the amount of substance in a given mass? moles=mass/Molar Mass
How do you work out the mass of a substance, given the amount? mass=moles x Molar Mass
What is the Empirical Formula? The simplest whole number ratio of atoms of each element present in a compound
What is the Molecular Formula? The number of atoms of each element that make up a molecule
How do you work out the Empirical formula from masses of each element? 1) Divide the mass of each element by the RAM; 2)divide by the smallest number 3) Check for SPECIAL ratio is whole numbers (so check for .25, .5, .33 or .66)
What is the empirical formula of a compound that has H 3.66%; P 37.8%; O 58.5%? 1)moles H=3.66/1=3.66; P=37.8/31.0=1.219 O=58.5/16=3.656; 2)divide by smallest H=3.66/1.219=3.00 P=1 O=3.656/1.219=3; 3)Special ratio? Not here so empirical is H3PO3
What is the empirical formula of a compound that has Pb 90.66%; O 9.34%? 1)moles Pb=90.66/207.2=0.437; O 9.34/16=0.584; 2)divide by smallest Pb=1; O=0.584/0.437=1.336; 3)Special ratio? Yes (.33 so multiply by 3) Empirical formula is Pb3O4
How do you work out the Molecular Formula from the masses of each element and the RMM? 1)Work out the empirical formula; 2)Work out the mass of the empirical formula; 3)work out the number of units in the molecular formula by dividing RMM by mass-of-empirical-formula; 4) Molecular formula is the number of units x empirical formula
What is Avogadro's Law? equal volumes of gases at the same temperature and pressure contain the same number of moles
How do you work out the amount of gas in a given volume? Number of moles=(PxV)/(RxT). Have you remembered to convert cm3 into m3? Celsius into Kelvins? Atm into Pa?
How do you work out the volume of gas, given the number of moles? Volume (in m3) = (nxRxT)/P. Have you remembered to convert cm3 into m3? Celsius into Kelvins? Atm into Pa?
How do you convert cm3 into dm3? Divide by 1000
How do you convert cm3 into m3? Divide by 1,000,000
How do you convert dm3 into cm3? Multiply by 1000
How do you convert m3 into cm3? Multiply by 1,000,000
What is the number of moles in 1080dm3 at 25C and 100kPa? Moles = (1.080m3x100,000Pa)/(8.31*298K) = 43.61 (did you remember to convert? Have you checked the significant figures?)
What is the volume of 0.25moles of Oxygen gas? Volume = 0.25 *24 = 6dm3; Have you remembered the units?
What is the mass in mg of 0.60dm3 of Nitrogen gas (N2) at room temp and 100kPa? Moles = (0.00060*8.31*298) / 100,000 = 1.48x10^-5; mass=molesxRMM=1.48x10^-5*28=0.41mg; Have you remembered to convert g into mg?
What is the volume of 1.282g of SO2 gas? Moles = mass/RMM=1.282 / 64.1= 0.02; volume=(nxRxT)/P = (0.02 x8.31x298)/100,000 = 0.495dm3; Have you remembered the units?
What is the mass of 0.500 moles of Fe2O3? RMM of Fe2O3 is 2x55.8+3x16.0=159.6 g.mol-1; Mass = 0.500 x 159.6 = 79.8g; Have you remembered the units? And significant figures?
What is the amount of Al2(SO4)3 in 10.269g? RMM of Al2(SO4)3=27.0x2+(32.1+4x16)x3=342.3 g.mol-1; Moles = 10.269 / 342.3 = 0.0300; Have you remembered the units? And significant figures?
What is the concentration of a solution? The amount of solute dissolved in 1dm3 of solvent
What is a standard solution? A solution of known concentration
How do you make up a standard solution? 1)weigh out the solute by difference;2)dissolve ina beaker and transfer into flask;3)rinse beaker, funnel and rod into flask;4)top up so that BOTTOM of meniscus is on graduation line
What is a molar solution? A solution with a concentration of 1 mole.dm-3
What is the unit of concentration? mol.dm-3
What is the amount of solution in 24.35cm3 of a 0.125M solution of Potassium Nitrate KNO3? moles = 24.35/1000 x 0.125 = 0.00304 or 3.04x10^-3
What is the concentration of 8.75x10^-3 moles in 50cm3 of solution? Concentration = 8.75x10^-3 / (50/1000)= 0.175 mole.dm-3; Have you remembered to convert cm3 into dm3?; Have you remembered the units?
What is the mass concentration of 3.56x10^-3 moles of H2SO4 dissolved in 25cm3 of solution? RMM H2SO4=2x1.0+32.1+4x16.0=98.1; Mass= 3.56x10^-3 x 98.1 = 0.349g; Concentration= 0.349 / (25/1000) = 13.96 g.dm-3; Have you remembered the units at each stage?
Al=27; O=16 nb of atoms of Aluminium x mass of Aluminium x 100 / Formula mass of Al2O3 = 2 x 27 x 100 /102 = 57%
work out the % by mass of Oxygen in Magnesium Sulfate, MgSO4; Mg=24; S=32; O=16 nb of atoms of Oxygen x mass of Oxygen x 100 / Formula mass of MgSO4 = 53.3%
work out the % by mass of Aluminium in Aluminium Oxide, Al2O3; Al=27; O=16 nb of atoms of Aluminium x mass of Aluminium x 100 / Formula mass of Al2O3 = 2 x 27 x100 / 102 = 57%
how do you work out the % by mass of an Element in a compound Work out the formula mass; then check the number of atom of your element; %by mass= number of atoms x RAM of element x 100 / formula mass
CHALLENGE: 239g of a Lead Oxide contains 32g of Oxygen. what is the empirical formula? Pb=207; O=16 You must have the mass of each ELEMENT: here mass of Lead is 239-32=207; Then as usual, Work out the moles of Pb=207/207=1 and moles of Oxygen=32/16=2; divide both numbers by the smallest: Pb=1/1=1 and O=2/1=2; the empirical formula is Pb1O2
Fe2O3 + 3 CO -> 2 Fe + 3 CO2; what mass of iron is produced when 80g of Iron(III)oxide is reacted? (Fe=56; O=16) Formula mass Fe2O3=56x2+3x16=160; moles Fe2O3=mass/RMM=80/160=0.5; ratio of BIG numbers says moles Fe= 2x molesFe2O3 =0.5x2=1; mass Fe=moles Fe x RMM=1x56=56g; remember to use BIG numbers in ratio ONLY
2 KClO3 -> 2 KCl + 3 O2; What mass of KCl is produced when 122.5g of KClO3 reacts completely? (K=39; Cl=35.5; O=16) RMM KClO3=39+35.5+3x16=122.5; form.mass KCl=39+35.5=74.5; moles KClO3=mass/RMM=122.5/122.5=1; ratio of BIG numbers says moles KCl= moles KClO3 =1; mass KCl=moles KCl x RMM=1x74.5=74.5g
CuCO3 -> CuO + CO2; What mass of Carbon Dioxide is produced when 12.4g of CuCO3 reacts completely? (Cu=64; C=12; O=16) Formula mass CuCO3=64+12+3x16=124; form mass CO2=12+16x2=44; moles CuCO3=mass/RMM=12.4/124=0.1; ratio of BIG numbers says moles CO2=moles CuCO3 =0.1; mass CO2=moles CO2 x RMM=0.1x44=4.4g
how do you work the mass of a compound, given the equation for the reaction and the mass of the reactant? set up the calculations under the equation: first line is mass; second line is formula mass; bottom line is moles; then go down (divide mass by formula mass); across (look at the BIG numbers); and up (multiply moles by formula mass)
In an experiment 2.4g of Magnesium is burned; and 3.6g of Magnesium Oxide is collected. What is the %yield of Magnesium Oxide? molesMg=2.4/24=0.1; ratio: molesMgO=molesMg; THEORETICAL mass of MgO=moles MgOxRMM=0.1*(24+16)=4.0. But only 3.6g is collected. What is the %yield?
A known mass of the hydrated salt, MCl2.6H2O was heated to constant mass. Mass of hydrated salt=5.51 g; Mass of anhydrous salt =3.01 g. Use the results to calculate the Mr of the anhydrous salt. Calculate the RAM and deduce the identity of the metal Mass H2O=2.5g; moles H2O=0.139; Moles MCl2=molesH2O/6=0.0231; RMM=5.51/0.0231=130;130=RAM+71; RAM=59; Metal=cobalt
A known mass of a hydrated carbonate X2CO3.10H2O was heated in a crucible to constant mass, Mass of hydrated carbonate =2.01 g; Mass anhydrous carbonate =0.74 g Identify the metal X. MassH2O=1.27g;MolesH2O=0.0705;moles anhydrous=0.0705/10=0.00705; RMM=mass/moles=0.74/0.00705=105.96=2*RAM+60;RAM=23;Metal=Na
1.6 g of methane was reacted with chlorine to make chloromethane. Cl2 + CH4 --> CH3Cl + HCl. The yield is 59%. What mass of product was collected? 1) Work out theoretical mass of CH3Cl=1.6/16*50.5=5.05g;2) mass collected = %yield * theoretical = .59*5.05=2.97g
the % yield in the following reaction is 80.0%. Mg + 0.5 O2 --> MgO. How much magnesium needs to be burned to make 30.0g of magnesium oxide? 1) Work out theoretical mass from %yield: theoretical=collected/yield=30/.8=37.5; 2)Down-across-Up to work out Mass reactant: Mass=37.5/40*24=22.5g
Created by: UrsulineChem