Chemistry 3/4 2013 Thread

[teletubbies_95]

Hi guys! I couldn't find the solution to this question! Can someone give me the answer , so I can compare? Thanks so much.
A concrete cleaner contains concentrated hydrochloric acid as the active ingredient. A 5.05 g sample of the product was diluted to 200.0 mL in a volumetric flask. Next, 20.00 mL of this
solution was transferred to a conical flask and titrated with a standard 0.100molL−1 sodium carbonate solution. The average volume required was 21.36 mL. The equation for the reaction is:
Na2CO3(aq) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l)
Calculate the mass of hydrochloric acid in 100 g of the cleaning product

Another question!

Equal masses of each of the following compounds is treated with excess HCl . Which product the greatest volume of CO2?
A) sodium carbonate
B) magnesium carbonate
C)pottasium carbonate
D) calcium carbonate
E) rubidium carbonate

What is the quickest way to do this question, instead of writing equations for each and working the answer out?

Thanks!!

[brightsky]

1) n(Na2CO3) = c*V = 0.100 * 0.02136 = 2.14*10^(-3) mol
Na2CO3(aq) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l)
so n(HCl) : n(Na2CO3) = 2 : 1
n(HCl)in 20.00 mL = 2*n(Na2CO3) = 2*2.14*10^(-3) = 4.27*10^(-3) mol
n(HCl)in 200.0 mL = 200.0/20.00 * 4.27*10^(-3) = 4.27*10^(-2) mol = n(HCl)originally
m(HCl) = n * M = 4.27*10^(-2) * 36.5 = 1.56 g
% by mass, HCl = 1.56/5.05 *100% = 30.9% (w/w)
so in 100 g, there would be 30.9 g of HCl.

2) you want the compound with the lowest molar mass. so MgCO3 should be correct answer.

[Edward21]

1) n(Na2CO3) = c*V = 0.100 * 0.02136 = 2.14*10^(-3) mol
Na2CO3(aq) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l)
so n(HCl) : n(Na2CO3) = 2 : 1
n(HCl)in 20.00 mL = 2*n(Na2CO3) = 2*2.14*10^(-3) = 4.27*10^(-3) mol
n(HCl)in 200.0 mL = 200.0/20.00 * 4.27*10^(-3) = 4.27*10^(-2) mol = n(HCl)originally
m(HCl) = n * M = 4.27*10^(-2) * 36.5 = 1.56 g
% by mass, HCl = 1.56/5.05 *100% = 30.9% (w/w)
so in 100 g, there would be 30.9 g of HCl.

2) you want the compound with the lowest molar mass. so MgCO3 should be correct answer.

Hey, I don't exactly understand why we want the compound with the lowest molar mass, could you explain that to me? 

[thushan]

Equal masses of each of the following compounds is treated with excess HCl . Which product the greatest volume of CO2?
A) sodium carbonate
B) magnesium carbonate
C)pottasium carbonate
D) calcium carbonate
E) rubidium carbonate

-----

You want the greatest volume of CO2. Hence, you'd want the greatest amount of CO2 (as n = pV/RT, so n and V are proportional assuming T and p are constant).

Now, we know that n(CO2) = n(metal carbonate), so we want the greatest amount of metal carbonate.

n(metal carbonate) = m/M

We know that the mass of each metal carbonate is the same; therefore, to maximise n(metal carbonate) we need the lowest molar mass.

[teletubbies_95]

Thanks so much rexxy and bright sky!

[Edward21]

Equal masses of each of the following compounds is treated with excess HCl . Which product the greatest volume of CO2?
A) sodium carbonate
B) magnesium carbonate
C)pottasium carbonate
D) calcium carbonate
E) rubidium carbonate

-----

You want the greatest volume of CO2. Hence, you'd want the greatest amount of CO2 (as n = pV/RT, so n and V are proportional assuming T and p are constant).

Now, we know that n(CO2) = n(metal carbonate), so we want the greatest amount of metal carbonate.

n(metal carbonate) = m/M

We know that the mass of each metal carbonate is the same; therefore, to maximise n(metal carbonate) we need the lowest molar mass.

Ohhhh I totally get it, the lower the number you're dividing by the more you'll get as a result, aka you'll get more n if m is being divided by a smaller M !!! 

[lzxnl]

E cell = E nought cell + RT/nF * ln Q

If you're given the standard electrode potential values, the temperature and the concentration of reactants and products at that specific time, you can use the Nernst equation to work out E cell, which in your case was 0.059 V. It's not part of VCE though, so no need to worry.

[9_7]

Hey guys im really confused with a situation!! If student 1 had 20mL in your titration with 0.1M of HCI or whatever, and student 2 had 40ml with the same concerntration, would there be any differences? My book says no, could someone please explain to me? Cheers

[Limista]

Hey guys im really confused with a situation!! If student 1 had 20mL in your titration with 0.1M of HCI or whatever, and student 2 had 40ml with the same concerntration, would there be any differences? My book says no, could someone please explain to me? Cheers

Your question is not very clear.Do you mean that the titre volume of HCl obtained by one student was 20mL, whereas the titre volume of HCl obtained by another student was 40mL?

[9_7]

Your question is not very clear.Do you mean that the titre volume of HCl obtained by one student was 20mL, whereas the titre volume of HCl obtained by another student was 40mL?

Sorry!! Um, the volume they put in the burette. So student 1 put 20mL in the burette and student 2 put 40mL in the burette

[Limista]

Okay so from what I gather, this is the situation:

Student 1 puts 20mL of HCl in burette of 0.1M concentration.

Student 2 puts 40mL of HCl in burette of 0.1M concentration.

There would be no difference in the amount in mole of HCl calculated as a result. Regardless of how much HCl is put in burette, titre volume of HCl in both reactions would be the same. You know the concentration of HCl (0.1M). You can then calculated the amount of HCl, and according to stiochiometric ratio of chemical equation, calculate the amount in mole of other reactant.

[Chazef]

Hey the heinemann textbook refers to the spiral structure of DNA being analogous to a spiral staircase where the phosphate-sugar backbone is the handrail and the complementary base pairs form the stairs, but I actually can't imagine being able to use a double-helix DNA-like staircase. Am I delusional or is it actually impossible to model a staircase after DNA?

[9_7]

Okay so from what I gather, this is the situation:

Student 1 puts 20mL of HCl in burette of 0.1M concentration.

Student 2 puts 40mL of HCl in burette of 0.1M concentration.

There would be no difference in the amount in mole of HCl calculated as a result. Regardless of how much HCl is put in burette, titre volume of HCl in both reactions would be the same. You know the concentration of HCl (0.1M). You can then calculated the amount of HCl, and according to stiochiometric ratio of chemical equation, calculate the amount in mole of other reactant.

Thanks =]

[Jayward]

Hey the heinemann textbook refers to the spiral structure of DNA being analogous to a spiral staircase where the phosphate-sugar backbone is the handrail and the complementary base pairs form the stairs, but I actually can't imagine being able to use a double-helix DNA-like staircase. Am I delusional or is it actually impossible to model a staircase after DNA?

the analogy just gets you to acknowledge the double helical natural of DNA and remember that the phosphate groups rim the outside edges of the macromolecule. I think it would be really hard to try and imagine being shrunk to microscopic proportions and then, try and climb a DNA molecule like a heap of stairs hahah. the analogy is very flawed, youre right. I think it would be better to refer to the DNA molecule as a zipper. The outer edges are the phosphodiester backbone and the inner zip itself is the nitrogenous bases and the H-bonds. A zip can either be coiled or uncoiled, much like DNA, at different stages of karyokinesis

[Chazef]

the analogy just gets you to acknowledge the double helical natural of DNA and remember that the phosphate groups rim the outside edges of the macromolecule. I think it would be really hard to try and imagine being shrunk to microscopic proportions and then, try and climb a DNA molecule like a heap of stairs hahah. the analogy is very flawed, youre right. I think it would be better to refer to the DNA molecule as a zipper. The outer edges are the phosphodiester backbone and the inner zip itself is the nitrogenous bases and the H-bonds. A zip can either be coiled or uncoiled, much like DNA, at different stages of karyokinesis

Thanks, makes sense