Embed Code - If you would like this activity on your web page, copy the script below and paste it into your web page.

Normal Size Small Size show me how

Normal Size Small Size show me how

# GCSE Calculations

### AQA GCSE chapter 04 calculations

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

work out the formula mass of NaCl. Na=23; Cl=35.5 | Formula Mass of NaCl = 1xMass Na + 1xMass Cl = 23 + 35.5 =58.5 |

work out the formula mass of NH4NO3 | 14*1 + 1*4 + 14*1 + 16*3 = 80 |

CHALLENGE work out the formula mass of (NH4)2SO4; N=14; H=1; S=32; O=16 | Formula mass of (NH4)2SO4= (massN x1 + mass H x4)x2 + massS x1 + massO x4= (14x1 + 1x4)x2 + 32x1 + 16x4 = 132 |

work out the formula mass of MgSO4; Mg=24; S=32; O=16 | Formula mass MgSO4 = MassMg x1 + MassS x1 + MassO x4= 24x1 + 32x1 + 16x4 = 120 |

how do you work out the formula mass of a compound? | multiply the mass of each element by the subscript that follows; then add the masses together; MgCl2 |

CHALLENGE 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/formula mass=80/160=0.5; ratio of BIG numbers says moles Fe= 2x molesFe2O3 =0.5x2=1; mass Fe=moles Fe x formula mass=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) | Formula mass KClO3=39+35.5+3x16=122.5; form.mass KCl=39+35.5=74.5; moles KClO3=mass/formula mass=122.5/122.5=1; ratio of BIG numbers says moles KCl= moles KClO3 =1; mass KCl=moles KCl x formula mass=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/formula mass=12.4/124=0.1; ratio of BIG numbers says moles CO2=moles CuCO3 =0.1; mass CO2=moles CO2 x formula mass=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) |

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? |

how do you work out the yield of a reaction? | yield = 100 x mass of product actually produced / maximum theoretical mass of product |

2 NaHCO3 --> Na2CO3 + H2O + CO2; if 16.8g of NaHCO3 are reacted and only 9.2g of Na2CO3 are produced, what is the percentage yield? | RFM(NaHCO3)=84; moles(NaHCO3)=16.8/84=0.2; moles(Na2CO3)=0.5xmoles(NaHCO3)=0.1; RFM(Na2CO3)=116; theoretical mass(Na2CO3)=0.1x116=11.6; percentage yield=100x9.2/11.6=79.3% |

CaCO3 --> CaO + CO2; if 200t of CaCO3 are reacted and 98t of CaO are produced, what is the percentage yield? | RFM(CaCO3)=100; moles(caCO3)=200/100=2; moles(CaO)=moles(CaCO3)=2; RFM(CaO)=56; theoretical mass(CaO)=2x56=112; percentage yield=100x98/112=87.5% |

Give 3 reasons why percentage yield are less than 100% | chemicals are lost during transfer from one place to another during separation; reaction is reversible; there may be side reactions producing other products |

What is a limiting reactant? | The reactant that gets used up first in a reaction; the reactant that is found in the smallest number of moles |

What is the relative atomic mass? | It is the average mass of one atom of an element compared to 1/12th of an atom of Carbon-12 |

Created by:
ursulinechem2