Chemistry Calculation- help!
I’ve just started A-Levels and my brain is still mush from the holidays (and recovering from a sickness), and I’ve been handed out chemistry calculations to do which were part of GCSE, but I don’t remember how to do it. I’ll put in the first question here:
Calculate the number of moles of the underlined species in the volume of solution stated.
25cm3 of 1.0 mol/dm3 of HCl
Can somebody briefly talk me through it because I’ve confused myself to no return at this point… thanks!
Calculate the number of moles of the underlined species in the volume of solution stated.
25cm3 of 1.0 mol/dm3 of HCl
Can somebody briefly talk me through it because I’ve confused myself to no return at this point… thanks!
Reply 1
Original post by Luciel_680
I’ve just started A-Levels and my brain is still mush from the holidays (and recovering from a sickness), and I’ve been handed out chemistry calculations to do which were part of GCSE, but I don’t remember how to do it. I’ll put in the first question here:
Calculate the number of moles of the underlined species in the volume of solution stated.
25cm3 of 1.0 mol/dm3 of HCl
Can somebody briefly talk me through it because I’ve confused myself to no return at this point… thanks!
Calculate the number of moles of the underlined species in the volume of solution stated.
25cm3 of 1.0 mol/dm3 of HCl
Can somebody briefly talk me through it because I’ve confused myself to no return at this point… thanks!
Recall the formula
Concentration in mol dm^-3 = (moles)/(volume in dm^3)
First and foremost, let’s rearrange the equation to make moles the subject, since they are what the question asks for
(Concentration in mol dm^-3) x (Volume in dm^3) = moles
So now we need to make sure everything has the right units. The concentration is given in mol dm^-3, which is correct, but the volume is given in cm^3, which won’t work.
So how would you convert the volume to dm^3 and then how would you use the rearranged equation to get your final answer?
Reply 2
Original post by Luciel_680
I’ve just started A-Levels and my brain is still mush from the holidays (and recovering from a sickness), and I’ve been handed out chemistry calculations to do which were part of GCSE, but I don’t remember how to do it. I’ll put in the first question here:
Calculate the number of moles of the underlined species in the volume of solution stated.
25cm3 of 1.0 mol/dm3 of HCl
Can somebody briefly talk me through it because I’ve confused myself to no return at this point… thanks!
Calculate the number of moles of the underlined species in the volume of solution stated.
25cm3 of 1.0 mol/dm3 of HCl
Can somebody briefly talk me through it because I’ve confused myself to no return at this point… thanks!
I perceived that Luciel is in serious trouble, so I propose the complete solution to the problem, even though I am breaking the forum rule of not solving a problem in detail.
To calculate the number of moles of HCl, you must use the formula:
Number of moles = Concentration (in mol/dm^3) x Volume (in dm^3) ==>(as UtterlyUseless69 already suggested)
Data and calculation:
1.
Concentration of HCl = 1.0 mol/dm^3
2.
Volume of solution = 25 cm^3 ==> you have to convert the volume from cm^3 to dm^3 ==> 0.025 dm^3 (because of 1 dm^3 = 1000 cm^3 = 1 L)==>(as UtterlyUseless69 already suggested)
3.
Number of moles = 1.0 mol/dm^3 x 0.025 dm^3 = 0.025 mol ==>(as UtterlyUseless69 already suggested)
Sandro
(edited 7 months ago)
Reply 3
Better yet, let’s give Luciel a very generic set of instructions to follow so that they can solve problems like this much more quickly.
1. Look at the data you are given and what variable it is you need to calculate. If you can think of a formula or formulae that link all of these variables, write them down and rearrange them as appropriate.
2. Look again at the data you have. Are the units of each variable you have appropriate to plug directly into the formula(e)? If not, convert them. If they are, then they’re ready to plug into the formula(e).
3. See what your calculator churns out. Include units with your answer and if it’s an enthalpy change, entropy change or ΔG value, make sure you include a sign.
1. Look at the data you are given and what variable it is you need to calculate. If you can think of a formula or formulae that link all of these variables, write them down and rearrange them as appropriate.
2. Look again at the data you have. Are the units of each variable you have appropriate to plug directly into the formula(e)? If not, convert them. If they are, then they’re ready to plug into the formula(e).
3. See what your calculator churns out. Include units with your answer and if it’s an enthalpy change, entropy change or ΔG value, make sure you include a sign.
(edited 7 months ago)
Reply 4
Original post by UtterlyUseless69
Recall the formula
Concentration in mol dm^-3 = (moles)/(volume in dm^3)
First and foremost, let’s rearrange the equation to make moles the subject, since they are what the question asks for
(Concentration in mol dm^-3) x (Volume in dm^3) = moles
So now we need to make sure everything has the right units. The concentration is given in mol dm^-3, which is correct, but the volume is given in cm^3, which won’t work.
So how would you convert the volume to dm^3 and then how would you use the rearranged equation to get your final answer?
Concentration in mol dm^-3 = (moles)/(volume in dm^3)
First and foremost, let’s rearrange the equation to make moles the subject, since they are what the question asks for
(Concentration in mol dm^-3) x (Volume in dm^3) = moles
So now we need to make sure everything has the right units. The concentration is given in mol dm^-3, which is correct, but the volume is given in cm^3, which won’t work.
So how would you convert the volume to dm^3 and then how would you use the rearranged equation to get your final answer?
Ah so I was getting the right answer the whole time… was just me being thick (and the answer not feeling very right to me). Thank you for helping
Reply 5
Original post by Luciel_680
Ah so I was getting the right answer the whole time… was just me being thick (and the answer not feeling very right to me). Thank you for helping
You weren’t being thick - if you were able to get the right answer, it shows you know what you are doing.
The questions by the sounds of it are meant to be a gentle warm up rather than an especially rigorous exercise. As such, the solutions quite probably are much easier than you are anticipating.
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