VCE Chemistry Question Thread

[eagles]

When copper (II) sulfate solution is stored in an iron container, the container gradually corrodes.
Write a balanced ionic equation to represent the reaction if the products of the reaction are copper and iron (II) sulfate solution. 

Answer: Cu²⁺ (aq) Fe (s) -> Cu (s) + Fe²⁺ (aq)

I would just to like to check whether we are allowed to dissociate ions in redox reactions?
And if so, is it because redox reactions are essentially ionic equations due to the transfer of electrons?

Thanks!

[Yacoubb]

When copper (II) sulfate solution is stored in an iron container, the container gradually corrodes.
Write a balanced ionic equation to represent the reaction if the products of the reaction are copper and iron (II) sulfate solution. 

Answer: Cu²⁺ (aq) Fe (s) -> Cu (s) + Fe²⁺ (aq)

I would just to like to check whether we are allowed to dissociate ions in redox reactions?
And if so, is it because redox reactions are essentially ionic equations due to the transfer of electrons?

Thanks!

Your half equations (i.e. reduction and oxidation half-equations) should be based on an ionic equation. Write your balanced equation, then write your ionic equation, and then, deduce your oxidation half equation and reduction half-equation.

[eagles]

Thanks for your help!

[eagles]

Just a question about sig figs:

A cylinder has a volume of 39.0 L filled with nitrogen gas to a pressure of 3.00 atmospheres at 23^oC. What mass of nitrogen does the cylinder contain?

Should the answer be to 2 or 3 sig figs? As there is a conversion of the 23 -> 296, do we now refer to the 296 as the given value and ignore the 23? Thanks!

[Yacoubb]

Just a question about sig figs:

A cylinder has a volume of 39.0 L filled with nitrogen gas to a pressure of 3.00 atmospheres at 23^oC. What mass of nitrogen does the cylinder contain?

Should the answer be to 2 or 3 sig figs? As there is a conversion of the 23 -> 296, do we now refer to the 296 as the given value and ignore the 23? Thanks!

2 sig figs. Its what you start with!

[eagles]

Cool thanks for checking (back of book has 3)

[thushan]

Nup, 3 sig figs.

Temperature in K = 23 + 273 (273 is the figure given in the Data Book as opposed to 273.0 etc) = 296 K.

When adding, its decimal places that matter, hence 296 K is the value to correct accuracy.

Since 296 happens to also have 3 sig figs, and so do all the other data that you use to multiply, the answer is to 3 sig figs.

In addition/subtraction you consider decimal places. In multiplication/division you consider sig figs.

[Yacoubb]

Nup, 3 sig figs.

Temperature in K = 23 + 273 (273 is the figure given in the Data Book as opposed to 273.0 etc) = 296 K.

When adding, its decimal places that matter, hence 296 K is the value to correct accuracy.

Since 296 happens to also have 3 sig figs, and so do all the other data that you use to multiply, the answer is to 3 sig figs.

In addition/subtraction you consider decimal places. In multiplication/division you consider sig figs.

Oh sorry eagles! Thanks for that thushan!

[eagles]

Dw yacoubb

Thanks for both of your responses!

[Rod]

Nup, 3 sig figs.

Temperature in K = 23 + 273 (273 is the figure given in the Data Book as opposed to 273.0 etc) = 296 K.

When adding, its decimal places that matter, hence 296 K is the value to correct accuracy.

Since 296 happens to also have 3 sig figs, and so do all the other data that you use to multiply, the answer is to 3 sig figs.

In addition/subtraction you consider decimal places. In multiplication/division you consider sig figs.

Hi Thushan,

Our teacher has told us to always use 3 sig figs when answering questions. But don't we look at the values given in the question, and then use the sigfigs from the question for our answer? Example the highest number of sig figs used in the question is 2, so then we have our answer in 2 sig figs. Or is my teacher right and and do we only use 3?

Thanks

[thushan]

Keep intermediate steps to correct sig figs - don't rely on the start of the question. However, use your calculator value for subsequent steps.

[eagles]

4.15 g tungsten is burned in chlorine and 8.95 g tungsten chloride (WCl₆) was formed.
Find the relative atomic mass of tungsten.

Please help haha thanks!

[Yacoubb]

4.15 g tungsten is burned in chlorine and 8.95 g tungsten chloride (WCl₆) was formed.
Find the relative atomic mass of tungsten.

Please help haha thanks!

We have 8.95g of WCl₆.
n(WCl₆) = 8.95/(6x35.5 + 183. =0.022555443 mol

There is one tungsten cation in tungsten chloride. Therefore n(WCl₆) = n(W)

Therefore, Mr of W = 4.15 / 0.022555443 mol = 183.9910615, to 3 sig figs, is 184.

Your answer is 184. But, why would it ask you to find the atomic mass of tungsten when you already have it on your periodic table?

[SocialRhubarb]

4.15 g tungsten is burned in chlorine and 8.95 g tungsten chloride (WCl₆) was formed.
Find the relative atomic mass of tungsten.

Please help haha thanks!

We begin with 4.15 g of pure tungsten, and finish with 8.95 g of tungsten chloride. All of the tungsten in the original sample is present in the tungsten chloride, so there must be 4.8 g of chlorine in the sample, as 4.15 g of the 8.95 g sample is tungsten.

4.8 g of chlorine is equivalent to 0.135 mol of chlorine. The ratio of tungsten to chlorine is 1 : 6, so the amount of tungsten is one sixth the amount of chlorine. Therefore, the amount of tungsten is 0.0225 mol.

We know that the amount of tungsten in the tungsten chloride is the same as the original amount of tungsten. Therefore, 0.0225 mol of tungsten has a mass of 4.15 g, and hence, by ratios, 1 mol of tungsten has a mass of 184.16 g, approximately 184.

Finally, the question asks for a relative atomic mass, so the answer must be in atomic mass units, so the relative atomic mass of tungsten is 184 atomic mass units.

We have 8.95g of WCl₆.
n(WCl₆) = 8.95/(6x35.5 + 183. =0.022555443 mol

There is one tungsten cation in tungsten chloride. Therefore n(WCl₆) = n(W)

Therefore, Mr of W = 4.15 / 0.022555443 mol = 183.9910615, to 3 sig figs, is 184.

Your answer is 184. But, why would it ask you to find the atomic mass of tungsten when you already have it on your periodic table?

You can't use the value for the molar mass of tungsten in the calculation of the relative atomic mass of tungsten. It's similar to mathematics - if a question asks you to prove a relationship, you cannot assume the relationship is true.

[eagles]

Awesome! Thank you