need help with chemistry question! (2 Viewers)

[kiimberly]

Hey guys, i need some help with the following question, i know the method of doing it, but it's the mole ratio part that i'm a bit confused about

1. C7NO3S can be determined by oxidising the sulfur to sulfate then precipitating it as barium sulfate. A0.118g tablet produced a 0.107g BaSo4. Calculate the percentage by mass of saccharine in the sugar tablet.

I know that first you work out the mass of the sample then you use the mole ratio and times it by the mass of saccharine, but it's the mole ratio that confuses me :L

Also i need help with this question:

a 6.67g sample of potato chips was blended with water and filtered. Excess silver nitrate was added. The dried precipitate had a mass of 0.245g. calcualte the percentage by mass of salt in the sample.

Is it okay if someone can go through the steps of working out gravimetric questions?

thank you and much help appreciated!

 

[girlworld_club]

God, why the hell am i doing chemistry next year??. I can't pronounce the name of these compounds (or whatever), let alone to solve them. Sorry for my inconvenience.

 

[LabGirl]

First, you need to know that the precipitate formed is silver chloride, AgCl. This is because chips contain salt, otherwise known as sodium chloride (NaCl). This NaCl dissolves and the Na+ ions and Cl- ions are left floating around in solution.

By adding silver nitrate, now we have two more ions floating around in the solution - Ag+ions and NO3- ions. Positive and negative ions combine together, sometimes forming a precipitate. All nitrate compounds are soluble so this means if NaNO3 forms, we are not going to know as it will all be dissolved! The only other product that can form is AgCl, which will always precipitate - don't worry about not knowing this, later on in the year you will be taught all the solubility rules and it will make a lot more sense!

Here is the reaction taking place and the molar ratio below.

Ag+ + Cl- -----> AgCl

1 : 1 : 1

This 1:1:1 ratio means that if we work out the number of moles in our 0.245g sample of AgCl then we know that we have exactly the same number of moles of Chloride.

Remember n=m/M

in your case n(AgCl) = m (0.245g) / Molar Mass of AgCl

Here is the next equation we need:

Na+ + Cl- -----> NaCl

1 : 1 : 1

Again we have a 1:1:1 ratio. This means the number of moles of chloride = number of moles of sodium chloride (salt!)

Next step is to say:

n(AgCl) = n(Cl-) = n(NaCl)

so:

m(salt in sample) = n(NaCl) x Molar Mass of NaCl

then to get the % in sample:

% of salt in sample = m(salt in sample) / total mass of sample (6.67g)

= ANSWER!

These sorts of questions get a lot easier with practice. Eventually you'll find them easy, trust me! The key is to keep practicing and do as many as you can. Plus don't give up! Chemistry is a great subject!!

 

[Anicca]

C7NO3S can be determined by oxidising the sulfur to sulfate then precipitating it as barium sulfate. A 0.118g tablet produced a 0.107g BaSo4. Calculate the percentage by mass of saccharine in the sugar tablet.

Saccharine is actually C₇H₅NO₃S...

Soooo the reactions was something like this:
1. C₇H₅NO₃S + 2O₂ --> C₇H₅NO₃ + SO^2-₄
2.SO^2-₄ + Ba²⁺ --> BaSO₄

Hence ...

Always find the no. of moles initially...

1. Find the no. of moles of barium sulfate ...
0.107 g divided by barium sulfate's molar mass (g/mol) =

Because they all have the same no. of sulfur atoms / molecule, the no. of moles of barium sulfate = the no. of moles of sulfate = the no. of moles of saccharine

2.Find amount of saccharine in the tablet ...
no. of moles of saccharine x molar mass of saccharine (g/mol) =

3.Pretty obvious what happens next

Hope this helped.