Chemistry 3/4 2013 Thread

[Stick]

The last step isn't necessarily an addition. Perhaps show your workings out and we can determine what's gone wrong.

[Homer]

I got 1.84456 g too, shouldn't it be 1.84?   

[Limista]

^ Homer: I haven't actually tried to solve the question, but I don't think you ought to stress too much about a 0.01 difference. It could be due to factors like using a different relative atomic mass from periodic table to solve the question, or maybe difference in number of sig. figs. used whilst calculating answer.

@ LOLs99: You should always give the answer to significant figures 

[KevinooBz]

Step 9
Calculate the total mass of BaSO4 precipitated from both reactions. m(BaSO4) = (0.821568 + 1.0229) g
= 1.84456 g
Step 10
Express the answer with the correct number of significant figures.
m(BaSO4) = 1.85 g
Solutions from heinemann, they incorrectly rounded but it doesn't really matter.

[LOLs99]

BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
n(Na2SO4)=0.500g/142.1g mol-1
                 =0.00352mol
n(BaSO4)=0.00352
m(BaSO4)= 0.821g

3BaCl2(aq) + Al2(SO4)3(aq) → 3BaSO4(s) + 2AlCl3(aq)
      n(Al2(SO4)3)= 0.500g / 342g mol-1
                          = 0.00146mol
n(BaSO4)= 0.00146mol x 3
               = 0.00438
m(BaSO4)=1.02g

Retain calc figures: 1.02278703g + 0.82125264g = 1.84g(3 sig fig)


** I use the vcaa exams periodic table and i have read before you need to use least dp when it's plus/minus in some lecture notes**

[LOLs99]

Oh if the worked solution shows incorrect rounding then i think im fine now. WHat about the sig fig thingy?
thats what i read from the notes.

Accuracy – “Sig Figs”
 

Here are the rules for quoting results in a nutshell:

Adding/Subtracting:   the result is only as accurate as the data with the least number of decimal places

            eg. 2.463 (3 dp) + 1.2 (1 dp) = 3.7 (1 dp)
            
            Least accurate value is to 1 dp.
            
Multiplying/Dividing:   the result is only as accurate as the data with the least number of significant figures

   eg. 1.265 (4 sf) x 1.31 (3 sf) = 1.66 (3 sf)

   Least accurate value is to 3 sf.

Exponentials:    10a = b
      
      If a is to p decimal places, b is to p significant figures
   
   eg. 10-4.76 = 1.7 x 10-5 (2 dp to 2 sf)
   
Logarithms:   log10 c = d
      
      If c is to p significant figures, d is to p decimal places

eg. log10 (1.74 x 10-4) = -3.759 (3 sf to 3 dp)

[Limista]

Oh if the worked solution shows incorrect rounding then i think im fine now. WHat about the sig fig thingy?
thats what i read from the notes.

Now that you mention that, I'm really not that sure anymore... 

[thushan]

BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
n(Na2SO4)=0.500g/142.1g mol-1
                 =0.00352mol
n(BaSO4)=0.00352
m(BaSO4)= 0.821g

3BaCl2(aq) + Al2(SO4)3(aq) → 3BaSO4(s) + 2AlCl3(aq)
      n(Al2(SO4)3)= 0.500g / 342g mol-1
                          = 0.00146mol
n(BaSO4)= 0.00146mol x 3
               = 0.00438
m(BaSO4)=1.02g

Retain calc figures: 1.02278703g + 0.82125264g = 1.84g(3 sig fig)


** I use the vcaa exams periodic table and i have read before you need to use least dp when it's plus/minus in some lecture notes**

Ah yes. You are adding a 3 dp number to a 2 dp number in your final step; your answer is to 2 dp (in this case, it happens to be in 3 sf).

[brightsky]

Ah yes. You are adding a 3 dp number to a 2 dp number in your final step; your answer is to 2 dp (in this case, it happens to be in 3 sf).

Just a quickie: so you're allowed to retain as many decimal places as you want until the final step?

[Mr Keshy]

Yeah, that's the best way to go about it

[thushan]

Just a quickie: so you're allowed to retain as many decimal places as you want until the final step?

In your calculator yes, but it's advisable to write each step to the correct accuracy, because (now this is hearsay from my teacher who is an examiner) you may get penalised for "inconsistent accuracy" in your working.

[Limista]

In your calculator yes, but it's advisable to write each step to the correct accuracy, because (now this is hearsay from my teacher who is an examiner) you may get penalised for "inconsistent accuracy" in your working.

By this do you mean not using the same number of significant figures throughout calculation?

[thushan]

By this do you mean not using the same number of significant figures throughout calculation?

Yup. Not sig figs per se, but accuracy (remembering when you add and subtract, you are dealing with decimal places).

[Limista]

Yup. Not sig figs per se, but accuracy (remembering when you add and subtract, you are dealing with decimal places).

So suppose the final answer requires 3 significant figures.

I decide to use 5 significant figures throughout calculation. I must use 5 significant figures for every value I calculate, before giving the final answer to 3 significant figures?

[thushan]

Assuming it's all multiplication and division, yes.

But I recommend you use 3 throughout (whilst keeping calculator values). This way, to determine the accuracy of your answer, you need only look at your previous line. This is a better way, because if you are doing a combination of additions and subtractions with multiplications and divisions, looking at the start of the qn for accuracy can be misleading.