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# BTEC calculations

Question | Answer |
---|---|

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