# Is the way I did this right (1 Viewer)

#### jazz519

##### Moderator
Moderator
Yes but if it is in short answer you would lose marks because you have not justified how you go from the 2nd last line to the last line.

Whenever you do things such an approximation you can't just do it without any explanation given. You have to write something like K is small therefore we can do the small x approximation and show 0.1 - x is approximately equal to 0.1 before doing that last line

#### specificagent1

##### Well-Known Member
Yes but if it is in short answer you would lose marks because you have not justified how you go from the 2nd last line to the last line.

Whenever you do things such an approximation you can't just do it without any explanation given. You have to write something like K is small therefore we can do the small x approximation and show 0.1 - x is approximately equal to 0.1 before doing that last line
oh i normally just solve the quadratic? this was how i was taught it? is this ok?

#### jazz519

##### Moderator
Moderator
oh i normally just solve the quadratic? this was how i was taught it? is this ok?
What I said is about that particular approach that was used in the image.

If you are solving the quadratic then there is no approximation being made and so there's no need to write that statement

• specificagent1

#### icycledough

##### Well-Known Member
oh i normally just solve the quadratic? this was how i was taught it? is this ok?
I don't really think it would be the best to solve the quadratic, as it can get very messy to work with decimals in quadratics.

• specificagent1

#### jazz519

##### Moderator
Moderator
I don't really think it would be the best to solve the quadratic, as it can get very messy to work with decimals in quadratics.
It depends on how they learnt it so @specificagent1 I wouldn't change this in the week prior to the HSC exam. Stick with the approach you have used so far as changing it last minute you might make mistakes with the other approach Although the approach with the approximations is quicker, it doesn't always work and so students sometimes may fall into the trap of applying it to scenarios where you cannot make it.

For example if the K = [A]^2 / [C] for a reaction and K = 100

and we do the calculation and end up with something like:
100 = x^2 / (0.1-x)^2

A lot of students will just blindly make the approximation here and that is why I made the comment earlier about you need to always state in which condition you can make the approximation.

Here if you did it, it is not valid because K is not small

In these types of scenarios if you had just learnt the quadratic approach you won't have to remember the limitation, so there is advantages to both methods

• CM_Tutor and specificagent1

#### specificagent1

##### Well-Known Member
It depends on how they learnt it so @specificagent1 I wouldn't change this in the week prior to the HSC exam. Stick with the approach you have used so far as changing it last minute you might make mistakes with the other approach Although the approach with the approximations is quicker, it doesn't always work and so students sometimes may fall into the trap of applying it to scenarios where you cannot make it.

For example if the K = [A]^2 / [C] for a reaction and K = 100

and we do the calculation and end up with something like:
100 = x^2 / (0.1-x)^2

A lot of students will just blindly make the approximation here and that is why I made the comment earlier about you need to always state in which condition you can make the approximation.

Here if you did it, it is not valid because K is not small

In these types of scenarios if you had just learnt the quadratic approach you won't have to remember the limitation, so there is advantages to both methods
thanks! i didnt even know there was another way of doing it

#### icycledough

##### Well-Known Member
It depends on how they learnt it so @specificagent1 I wouldn't change this in the week prior to the HSC exam. Stick with the approach you have used so far as changing it last minute you might make mistakes with the other approach Although the approach with the approximations is quicker, it doesn't always work and so students sometimes may fall into the trap of applying it to scenarios where you cannot make it.

For example if the K = [A]^2 / [C] for a reaction and K = 100

and we do the calculation and end up with something like:
100 = x^2 / (0.1-x)^2

A lot of students will just blindly make the approximation here and that is why I made the comment earlier about you need to always state in which condition you can make the approximation.

Here if you did it, it is not valid because K is not small

In these types of scenarios if you had just learnt the quadratic approach you won't have to remember the limitation, so there is advantages to both methods
Correct me if I'm wrong, but is the threshold to be able to use the approximation when K > 1000 or K < 0.0001 (So 10^-4 < K < 10^4)?

#### jazz519

##### Moderator
Moderator
Correct me if I'm wrong, but is the threshold to be able to use the approximation when K > 1000 or K < 0.0001 (So 10^-4 < K < 10^4)?
Yeah you can use it on the very large ones or very small ones but your last inequality isn't correct it's saying K has to between those things. The first version of the inequality is fine (but change K > 10000)

#### icycledough

##### Well-Known Member
Yeah you can use it on the very large ones or very small ones but your last inequality isn't correct it's saying K has to between those things. The first version of the inequality is fine (but change K > 10000)
Oh yh, my bad. It was supposed to be an either side inequality, don't know why I contradicted myself there.

• jazz519