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Calculate the number of atoms in each of the following;
(i) 52 moles of Ar 
(ii) 52 u of He
(iii) 52 g of He.


(i) 52 moles of Ar.


(ii) 52u of He.
Atomic mass of He = 4u
i.e. 4u of He = 1 atom He
 52 u He 


(iii) 52 g of He.
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Calcium carbonate reacts with aqueous HCl to give CaCl2 and CO2 according to the reaction,
 CaCO subscript 3 left parenthesis straight s right parenthesis space plus space 2 HCl space left parenthesis aq right parenthesis space rightwards arrow space space CaCl subscript 2 left parenthesis aq right parenthesis space plus space CO subscript 2 left parenthesis straight g right parenthesis space plus space straight H subscript 2 straight O left parenthesis straight l right parenthesis
What mass of CaCO3 is required to react completely with 25 mL of 0·75M HCl?


1000 mL of HCl contains 0·75 mol
                                 = 0.75 x 36.5
                                 = 27.375 g

   25 mL of HCl contains
                                        = 0.684 g
The reaction is
CaCO3(s)    +   2HCl(aq)    CaCl2(aq) + CO2(g) + H2O(l)
1 mol                 2 mol
40+12+48          2 x 36.5
 = 100 g              = 73 g
?                        0.684 g
    Now 73 g of HCl are required to react with 100g of CaCO3
     
0.684 g of HCl are required to react with

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