Atomic Structure Practice Answers

 

Q1.          
i)   K+  protons = 19; neutrons = 20; electrons = 18 (1+ charge, so 1 electron removed)
ii)  H+  protons = 1;  neutrons = 0;  electrons = 0  (1+ charge, so 1 electron removed)
iii) H-  protons = 1;  neutrons = 0;  electrons = 2  (1- charge so 1 electron added)
Q2.
i)  37Cl-         atomic number of 17 may also be given
ii) 24Mg2+        atomic number of 12 may also be given
Q3.    
relative atomic mass =  (78.6 x 24) + (10.1 x 25) + (11.3 x 26) = 24.3
                         100           100           100   
Q4.
Isotope   rel. abund.    fract. abund. x mass number
20Ne         90.0                  0.9 x 20    = 18.0
22Ne         10.0                  0.1 x 22    = 2.2
                                      Ar     =   20.2         

 

Q5.
100% - 92% = 8% so there is 8% of the other isotope
Let its mass number be m.
Then  ((8 x m) + (92 x 7)) ÷ 100 = 6.92   ⇒ 8m = 48 ⇒ m=6  so  6Li
Q6.
i)   propanol         = (3x12) + (8x1) + 16       = 60
ii)  sulphur dioxide  = 32.1 + (2x  16)           = 64.1
iii) nitrogen gas     = 2x14                      = 28
iv)  trichloromethane = (3x35.5) + 12 + 1         = 119.5
v)   ammonium nitrate = 14 + (4x1) + 14 + (3x16)  = 80
Q7.
i)   HNO3                        63.0 g mol-1
ii)  Na2CO3.10H2O                286.0 g mol-1
iii) (NH4)2SO4.Fe2(SO4)3.24H2O    964.2 g mol-1
Q8.
i)   0.1 mole of Zn       0.10 x 65.4       6.5g to 2sf
ii)  1 mole of KHCO3      1 x 100.1g        1.0 x 102 to 2sf
iii) 0.05 moles of Al2O3  0.05 x 102        5.1g to 2sf
Q9.
i)   79.9g of Br2         79.9 ÷ 79.9   = 1          1.0 to 2sf
ii)  8.5g AgNO3           8.5 ÷ 169.9   = 0.05003    0.050 to 2sf
iii) 200kg of NaOH        200000 ÷ 40   = 5000       5.0 x 103 to 2sf
Q10.
i) How many molecules are there in 79.9g of Br­2?  3.01 x 1023
ii) How many Br atoms are there in 159.8g of Br2? 1.204 x 1024
iii) How many oxygen atoms in 8.5g AgNO3?  = 3 x 0.05003 x NA  = 9.035 x 1022
iv) Mr of average chain = (12 + 1 + 1) x 100,000  = 1,400,000
    ⇒ 1 mol of chains weighs 1,400,000g  (definition of molar mass)
    ⇒ 1 chain weighs 1,400,000 ÷ NA      = 2.32x10-18 g
    ∴ chains in 2g bag = 2 ÷ 2.32x10-18  = 9x1017 chains
Q11.
i)   moles of H2 = 300 ÷ 2 = 150    volume = 150 x 24    = 3,600 dm3 
ii)  moles = 100 ÷ 24,000                               = 0.00417 mol.
iii) molar mass of CO2 = 44g mol-1
     molar gas volume = 24,000 cm3
     density = mass ÷ volume    = 44 ÷ 24,000           = 0.00183 gcm-3