past hsc chem q help! (1 Viewer)

[mecramarathon]

Graaahhh this was in the 2009 HSC chem, can anyone plz explain to me how to do this!? :angry:


The nitrogen content of bread was determined using the following procedure:

• A sample of bread weighing 2.80 g was analysed.
• The nitrogen in the sample was converted into ammonia.

• The ammonia was collected in 50.0 mL of 0.125 mol L<SUP>−1 hydrochloric acid. All of the ammonia was neutralised, leaving an excess of hydrochloric acid.
• The excess hydrochloric acid was titrated with 23.30 mL of 0.116 mol L−1 sodium hydroxide solution.

(a) Write balanced equations for the TWO reactions involving hydrochloric acid.
(b) Calculate the moles of excess hydrochloric acid.
(c) Calculate the moles of ammonia.
(d) Calculate the percentage by mass of nitrogen in the bread.
</SUP>

 

[x jiim]

Umm, I'll try..

a] HCl + NH3 --> NH4+ + H-
and HCl + NaOH --> NaCl + H2O

b] molar ratio HCl:NaOH = 1:1
therefore equal moles of HCl and NaOH
moles NaOH = 0.116 x 0.0233 = 0.0027028
so 0.0027 moles excess HCl [not sure about sigfigs]

c] total amount HCl present is 0.125 x 0.05 = 0.00625moles
therefore moles HCl reacted is 0.00625-0.0027028 = 0.0035472moles
molar ratio HCl:NH4 = 1:1
so 0.00355moles NH4 produced

d] In 0.00355 moles NH4 there are 0.00355 moles of N
so mass of N present in bread was 0.00355x 14.01
=0.049696272..
=0.05g
which is about 1.77% of the bread mass

is that it? =\

 

[adomad]

(a) Equation one:
Equation two:

(b) to find the moles of excess HCl, we need to find the moles of NaOH present in the 23.30 mL of 0.116 mol L−1 sodium hydroxide solution-->



(c) To find the moles of ammonia, we must find the moles of HCL in the 50 mL sample that reacted completely with the ammonia...->




(d) becuase they want percentage by mass, we must find the mass of ammonia->

 

[mecramarathon]

gosh thanks!,

xjiim just a slight error in reaction with ammonia and hydrochloric reaction, should be NH^4 + Cl-

and slight error for adomad, finding the % by mass for the nitrogen, not the ammonia so should be 1.78% but meh


i got up to the last q and kept thinking nitrogen was N2 LOL

ty!

 

[adomad]

Thanks for picking that up.. i thought the question was linear and a b c and d were like a step after step... DW.NP