Tips for Chemistry Practicals (P3)

[Majed Kaaki]

can someone please help me ! abt the accuracy questions that they always ask abt the burrette and pipete . and mass ... i always get mixed up

 

[Apple Ipad64]

can someone please help me ! abt the accuracy questions that they always ask abt the burrette and pipete . and mass ... i always get mixed up

its like the error in burette or pipette is half the least count. The least count of burette is 0.1cm^3 so the error is +- 0.05. bt since we take an initial n a final value to get a titre, we multiply the error by 2 i.e +-0.1.
Same applies to thermometers.
in pipette u dont multiply it by 2 since u just measure 1 final volume

 

[CaptainDanger]

its like the error in burette or pipette is half the least count. The least count of burette is 1cm^3 so the error is +- 0.5. bt since we take an initial n a final value to get a titre, we multiply the error by 2 i.e +-1.
Same applies to thermometers.
in pipette u dont multiply it by 2 since u just measure 1 final volume

Errm... It is .1 cm^3, talking about burette least count... Which makes the error ± .05 cm^3 for any individual value...

 

[user]

Errm... It is .1 cm^3, talking about burette least count... Which makes the error ± .05 cm^3 for any individual value...

opps. my mistake. yea..i was thinking abt sumthyn else.

Aoa wr wb!

Quote from the syllabus:

 

[Apple Ipad64]

Aoa wr wb!

Quote from the syllabus:

View attachment 9094

 

[Majed Kaaki]

its like the error in burette or pipette is half the least count. The least count of burette is 0.1cm^3 so the error is +- 0.05. bt since we take an initial n a final value to get a titre, we multiply the error by 2 i.e +-0.1.
Same applies to thermometers.
in pipette u dont multiply it by 2 since u just measure 1 final volume

thank uu for the helpp ! thanks god they ddnt bring it hard anywayzz abt the errorss i did great in todays exam el7amdellah

 

[Apple Ipad64]

thank uu for the helpp ! thanks god they ddnt bring it hard anywayzz abt the errorss i did great in todays exam el7amdellah

oh im glad

 

[Haider2011]

I'll upload some pictures as soon as I get my journal from the school, explaining how to draw the tables and write the observations.

plz upload photo

 

[prekshya]

any quesses for chemistry paper 34?????

 

[basselsllw21]

i have no idea how to draw the graph.... any notes regarding that?

 

[MindStealth]

Since I don't have the time to actually draw figures and explain I'll just write some guidelines for drawing the graph.

• Think carefully before choosing scales for the axes, you need to keep in mind that the scale needs your values to cover at least half of the page and should is sensible, by that I mean you should be able to easily find values on the axes without needing to calculate what value some point on the axes represent.
• Sharp your pencil, the last thing you want is to lose marks because your line was too thick.
• Double check when marking the points, 1 mistake and in some cases you might mess up the whole graph.
• "Line of best fit." Doesn't necessarily mean that its a straight line. Although rarely, it could be a curve too that fits most of the points.
• In graphs of experiments like heating and then cooling, you need to draw two lines, 1 representing cooling and the other one representing heating instead of a curve.
• Whenever they ask you to find gradient, ALWAYS draw a triangle, and the bigger the triangle, the more accurate your gradient.
• If you have doubt about your gradient, just take any point that is close to the line and find (y/x), this value should be very close to the gradient found using the triangle. But remember this only works when the graph is starting from origin or near it.
• Try to never break your graph but if you do, REMEMBER that the y-intercept needs to be calculated using the equation "y = mx + c" where m is the gradient you found, c is the y-intercept and can be found using the y and x value of any point on the lane.
• Try to keep your graph clean and visible. If you need multiple lines to find gradient and other value, make sure they're distinguishable.
• ALWAYS write what each axis represents with its unit.
• To determine which quantity goes on the x and y axis, you need to find which quantity is dependent and which one is independent. Independent is usually the one you are changing and dependent is the one you measure.
• If you draw multiple graphs, make sure to label them.
• Don't spend more than 15 minutes on the graph. The graph is usually simple, and if its not then you've done probably something wrong and instead of wasting time on it, try to score marks in other parts first and then come in the end to figure out what's the problem with the graph.
• If you're good at salt analysis, then do it first and finish it fast so you can do the graphing without any tension.
• REMEMBER even if you draw the graph wrong due to some wrong measurements or something, don't lose hope on it, you can still workout the rest of the questions for the wrong graph and score most of those marks like gradient and stuff.
• When they ask you to prove some relationship like T=kV or something, write that since the graph is a straight line, the value of the K is constant and therefore the relationship is true...unless emm...your line is not straight.
• If you don't have time to draw the graph, try to do as much as you can like plotting two or three values and then making the line instead of all the points or even writing the quantities on the axes can gain your marks.

That's pretty much it, Good Luck for your exams. I had variant 1 and was pretty easy.

 

[Ashique]

What's the uncertainty of a stop watch?

 

[MindStealth]

What's the uncertainty of a stop watch?

Well the uncertainty depends on how big of a value you are finding but I think what you want to ask is the error in a stop watch.
Error in any instrument is half of the smallest unit you can read on it. For instance, the stopwatch we had in the lab had the smallest unit as a 0.01s so the error in the readings read would be +/- 0.005s.
Now you can find the uncertainty using the following formula, it might give you an understanding why it depends on the size of the value you measure.
Ex. If I measure a value of 50s using the same stopwatch as mentioned before with the error 0.005s. Then,
Uncertainty = [(error)/(my measured value)]*100
= [0.005/50]*100
= 0.01 percent.

 

[Nishii]

Hey could you please share some examples of how to plan an experiment in p5?

 

[aleezay]

MindStealth, don't you take the uncertainty of a stopwatch to be equivalent to the value of human reaction time error? Which is 0.2-0.4s I believe... because I've seen this in a number of marking schemes for physics practicals. Wouldn't the same apply for chemistry? :/

 

[MindStealth]

MindStealth, don't you take the uncertainty of a stopwatch to be equivalent to the value of human reaction time error? Which is 0.2-0.4s I believe... because I've seen this in a number of marking schemes for physics practicals. Wouldn't the same apply for chemistry? :/

This is going to be a little hard to explain but this is how it works, you(Student) are supposed to take the standard uncertainity in timers. The examiner on the other hand takes extra for human error because there's a huge chance you will get it on a stopwatch. So basically you need to think that you didn't make any human error due to reaction times, so you will just take the error that is due to the stopwatch itself, the examiner will take care of the errors caused by your reaction times.

 

[A*(a*)]

This is going to be a little hard to explain but this is how it works, you(Student) are supposed to take the standard uncertainity in timers. The examiner on the other hand takes extra for human error because there's a huge chance you will get it on a stopwatch. So basically you need to think that you didn't make any human error due to reaction times, so you will just take the error that is due to the stopwatch itself, the examiner will take care of the errors caused by your reaction times.

Before this, I had no doubt that error in a stopwatch is to be taken as 0.2s or something bit more or less, As per a guide of physics released by CIE, which means that there is no question about is authenticity,

· The uncertainty in a measurement can sometimes be larger than the smallest interval that can be measured by the measuring equipment For example, a stopwatch can measure time to a hundredth of a second, but human reaction times will mean that the uncertainty in the reading given by a stopwatch is TYPICALLY 0.2-0.3
As far as I can understand from this is, that in calculating uncertainity we are required to take 0.2s error straightaway. Does not 0.005 seems odd? If any other person would have said this, I would have ignored but seeing your notes, I thought i should share.

 

[A*(a*)]

Errm... It is .1 cm^3, talking about burette least count... Which makes the error ± .05 cm^3 for any individual value...

See my previous post, what do you say 0.2 or 0.005 ? because I have really worked hard to understand it :/

 

[CaptainDanger]

^0.2

 

[A*(a*)]

^0.2

Its so hard to decide the uncertainities of different instruments, Can I get them all for all the instruments to be used in labs?