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Questions on mass of precipitate in a chemical reaction

1. The chlorine in a .63g sample of chlorinate pesticide, DDT (C14H9Cl5), is precipitated as silver cloride. What mass of silver chloride is formed?

2.A precipitate of Fe2O3, of mass 1.43g, was obtained by treating a 1.5L sample of bore water. What was the concentration of iron, in molL-1, in the water?

3. A .5g sample of sodium sulfate (Na2SO4) and .5g sample of aluminium sulfate (Al2(SO4)3 ) were dissolved in a volume of water and excess barium chloride added to preciptate barium sulfate. What was the totoal mass of barium sulfate produced?

i keep getting 1.02g for this one but the answer is 1.85g

Silver chloride can only be precipitated from a solution that contains Cl- ion.  The DDT itself is very insoluble and the Cl atoms are held by means of covalent bonds which means that a solution of DDT would contain only a tiny amount of DDT and of that none of the Cl atoms would be in a form where they could be precipitated by Ag+

However, if the question is assuming that the DDT has been decomposed and that the products of the decomposition reaction include all of the Cl atoms in the DDT sample, and that those atoms have been converted into Cl ions. Here is how you would proceed:

From the formula, determine the % composition of Cl in the compound.  (% = 5 x  mass of Cl/ mole mass of DDT  x100)

Using the % Cl, determine the mass of Cl in the .63 g sample.

Find the moles of Cl.

Assume that the Cl atoms are somehow converted into Cl- ions, use the balanced equation for the reaction of Ag+ and Cl- to obtain them moles of AgCl:
                               Ag+ (aq)      +   Cl- (aq)   -->  AgCl (s)
 
                               (moles of Ag+ and moles of AgCl are in a 1:1 ratio)
Convert the moles of AgCl to mass.

2)  Convert 1.43 g of Fe2O3 into moles of Fe2O3 using the molar mass of Fe2O3

Since the formula Fe2O3 specifies that there are two moles of Fe for each formula unit: moles of Fe = 2 x moles of Fe2O3.

Molarity of Fe  = moles of Fe / volume of solution in Liters              (volume = 1.5 L in this case)

3) Determine moles of sodium sulfate and moles of Aluminum sulfate.
   
Determine moles of SO4-2 ions:    moles of Na2SO4    +   3 x moles of Al2(SO4)3    (My quess is that you forgot to multiply by 3 in this step)

Since Ba+2 and SO4-2 combine in a 1:1 ratio, moles of BaSO4 formed will equal moles of SO4-2 ions

Convert moles of BaSO4 to mass.

thanks spock , for going to all that trouble, yea i get it now




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