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# BTEC calculations

What is the standard for Relative Mass, to which all other masses of atoms are compared to? 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; RAM= sum of (abundancexmass) / total abundance; (19.77x10+80.23x11)/100
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 is the Molar Mass? The mass of a mole of a substance.
What is the unit of Molar Mass (aka Relative Formula 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
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 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 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?
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? (1)molesMg=2.4/24=0.1; (2)ratio: molesMgO=molesMg; (3)THEORETICAL mass of MgO=molesMgO x RMM=0.1*(24+16)=4.0. (4)But only 3.6g is collected. %yield=collected/theoreticalx100=3.6/4.0x100=90%
Created by: ursulinephys