[shama]

hello everyone
I'm a new member in the chemical forum and i registered because I need a great help in my subjects .. I study pharmacy ,, this is my second year in the faculty ,,,
I have some difficult problems and I need your help ... please reply soon because I have an exam on 18/11.. the problems are all about gravimetry ... they are:
1) A 5.000 g sample of a pesticide was decomposed with metallic sodium in alcohol, and the liberated chloride ion was precipated as AgCl. express the results of this analysis in terms of the precentage of 1,1,1-trichloro-2,2-bis-(parachlorophenyl)ethane DDT (C₁₄H₉Cl₅)(fw=354.5) based upon the recovery of 0.1606 g of AgCl.

2) A 1.008 g sample that contained NH₄NO₃, (NH₄)₂SO₄ ,and inert materials was dissolved in suficient water to give exactly 500 ml of solution, A 50.0 ml aliquot of this solution yeilded 0.3819 g of (C₆H₅)₄BNH₄ (fw=337) when treated with an ecxess of sodium tetraphenylboron (C₆H₅)₄BNa, A second 50.0 ml aliquot was made alkaline and heated with Devrada's alloy (50% Cu,45% Al,5% Zn) to convert the No₃^- to NH₃
No₃^- + 6 H₂O+ 8e___NH₃ + 9OH^-
the NH3 produced in this reaction, as well as that derived from NH4⁺ , was distilled and collected in dilute acid. Treatment of the distillate with (C₆H₅)₄BNa yielded 0.5996 g of C₆H₅)₄BNH₄ . Calculate the percentages of NH₄NO₃, (NH₄)₂SO₄ in the sample.

please explain with answer
thanks alot
you are the best

[AWK]

These are, in fact, relatively simple stoichiometric problems. Show your attempts.

[shama]

in the second q.
I say that 5o ml of soln. contain 0.3819 g of (C₆H₅)₄BNH₄ so in the total soln 500 ml there are 38190 g of the cpd.. and I tried to calculate the grams of (C₆H₅)₄BNa according to the equation I $uppo$ed
(C₆H₅)₄BNa+NH₄⁺ gives Na⁺ + (C₆H₅)₄BNH₄
then I calculated the same In the other 50.0 ml  then I didn't know how to continue??

in the first q. I don't know how to begin because I don't understand the relation of C₁₄H₉Cl₅) in the question...
that's all

[shama]

hello again
i really need help,,, yesterday evening I tried to solve the problems and I have answers ,, about the 1^st q. ,,,
I get no. of moles NH₄⁺ in the second 5o ml .. 1.7792*10^-3 , and it equals the no. of moles NH₄NO₃ , then i calculated its percentage by getting its wt 0.42409 then 14.127%
the no. of moles NH₄⁺ in the first 50 ml is 1.3323*10^-3 ,,, so in the total 5oo ml equal 0.1332344 moles ,,,  then
1.7792*10^-3 _ 0.1332344 = 0.11544213 ... and these are no. of moles for NH₄⁺ in (NH₄)₂SO₄
which equals twice no. moles of (NH₄)₂SO₄ ,, then i get its wt. then percentage 75.6672% ,, is this true???

about second q. i calculated no. mols cl^- in .1606 g AgCl ,, then In 5 gram which is the grams of sample
I calculated the no. mols of the cpd given in q. by saying 5 mols cl equal 1 mol of cpd ..
and get it wt by multiplying with its fw,, then the % is 49.4697%

plz can any one help me in revising my steps... I tried herd to solve them... and If my ask or my whloe topic is against rules,,, can any one tell me??
thnx alot

[Borek]

In the 1^st question - AgCl contains all chlorine that was present in DDT. You can calculate number of moles of Cl. If you look at DDT formula you will see there are 5 Cl atoms per DDT molecule, or 5 chlorine moles per DDT mole - use this information to calculate number of moles of DDT (from known amount of chlorine). Knowing number of moles of DDT calculate its mass (you are even given molar mass) - and you are done.

2^nd question is in a way similar to the AgI and AgCl problem you have posted separately - you have two types of information that'll let you write two equations in two unknowns. First experiment gives information about sum of all NH₄⁺ cations, second gives that PLUS amount of NO₃^-.

I get no. of moles NH₄⁺ in the second 5o ml .. 1.7792*10^-3 , and it equals the no. of moles NH₄NO₃

You are alreaydy wrong here - 1.779x10^-3 is nitrogen from all possible sources - ammonium from ammonium sulfate, ammonium form ammonium nitrate AND ammonium prepared by reduction of nitrate (from ammonium nitrate).

When posting please avoid using series of punctuation marks and o instead of 0, it makes your posts hard to read - and you want them easy to read, you want them answered

It is OK to ask these questions - just always show how you tried to approach them. Note that - so far - they can be all asked in General Chemistry forum, while they may look pretty analytical, they are in fact just stoichiometry.

[shama]

hi...
thank you for your explaination... and really sorry if I did wrote some formulas wrong , I know them very well ,, it's just a mistake during typing
but I'm sorry if there are some wrong words written or grammer,,, I'm not an american or english girl .. I'm an arabic girl... palestinian if you know occupied Palestine.. so accept my excuse??

however thanks alot,, you help me and I appreciate that