Chemistry calculations help please

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blossomx
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#1
Report Thread starter 7 years ago
#1
So I'm currently doing an undergrad Biology module which involves lots of Chemistry calculations. I thought doing a-level Chemistry would help but the basic conc=moles/volume and mass = Mr/moles is no longer sufficient.

I was wondering if anyone had any resources they recommended to help with the calculation side of things. Some examples of questions we need to be able to are things like:

- What volume of this solution of 1g NaCl in 100ml water would you use to make 500mL of a 10microgram/mL solution.
- You dilute the entire 500mL in 5L of water, what is the final conc of NaCl?
- What is the molarity of this NaCl conc?

We were given these questions to practice ready for an assignment and weren't told how to complete them and I have no idea. I've looked on Chemguide (my saviour for a-level chemistry lol) but to no avail.

Any advice or suggestions please?
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charco
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#2
Report 7 years ago
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(Original post by LeaX)
So I'm currently doing an undergrad Biology module which involves lots of Chemistry calculations. I thought doing a-level Chemistry would help but the basic conc=moles/volume and mass = Mr/moles is no longer sufficient.

I was wondering if anyone had any resources they recommended to help with the calculation side of things. Some examples of questions we need to be able to are things like:

- What volume of this solution of 1g NaCl in 100ml water would you use to make 500mL of a 10microgram/mL solution.
- You dilute the entire 500mL in 5L of water, what is the final conc of NaCl?
- What is the molarity of this NaCl conc?

We were given these questions to practice ready for an assignment and weren't told how to complete them and I have no idea. I've looked on Chemguide (my saviour for a-level chemistry lol) but to no avail.

Any advice or suggestions please?
It's just simple sums.

There are 1000 mg in a gram
There are 1000 micrograms in a mg

There are 1000 mL in litre.

Go for the target solution first and work out the total micrograms per volume required. Then work backwards..
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blossomx
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#3
Report Thread starter 7 years ago
#3
(Original post by charco)
It's just simple sums.

There are 1000 mg in a gram
There are 1000 micrograms in a mg

There are 1000 mL in litre.

Go for the target solution first and work out the total micrograms per volume required. Then work backwards..
Thank you, I've had a go at the questions and was wondering if you would possibly be able to look through them please? I understand if you don't have time to, though. Also what is the difference between concentration and molarity? I always thought that they were interchangeable?

How much NaCl do you need to weigh out to prepare 1 L of a 1M solution of NaCl?
- Moles = conc x vol
- Moles = 1
- Mass = Mr x Mole
- Mass = 58.44 x 1
- Mass = 58.44g

How many millimoles of NaCl are there in 1 gram?
- Moles = Mass/Mr
- Moles = 1/58.44
- Moles = 0.017mol
- Milimoles = 17.11mol

You dissolve 1 gram of NaCl in 100mL of water, convert this conc to mg/mL
- 1000/100 = 10mg/mL

What volume of this solution is required in order to make 500mL of a 10microgram/mL solution?
- 10microgram/mL solution equals 0.01mg/mL
- mg/mL x mL = mg
- 0.01 x 500 = 5mg required
- mg / mg/mL = mL
- 5 / 10 = 0.5mL

Dilute the entire 500mL of solution in 5L of water, what is the final conc of NaCl?
- 500ml contains 5mg of NaCl
- Moles of NaCl: 5x10^-3 / 58.44 = 8.56x10^-5mol
- Conc = moles / total volume
- Conc = 8.56x10^-5mol / 5.5 = 1.55x10^-5moldm-3

What is the molarity of NaCl in this solution?
1.55x10^-5moldm-3.
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charco
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#4
Report 7 years ago
#4
(Original post by LeaX)
Thank you, I've had a go at the questions and was wondering if you would possibly be able to look through them please? I understand if you don't have time to, though. Also what is the difference between concentration and molarity? I always thought that they were interchangeable?

How much NaCl do you need to weigh out to prepare 1 L of a 1M solution of NaCl?
- Moles = conc x vol
- Moles = 1
- Mass = Mr x Mole
- Mass = 58.44 x 1
- Mass = 58.44g


How many millimoles of NaCl are there in 1 gram?
- Moles = Mass/Mr
- Moles = 1/58.44
- Moles = 0.017mol
- Milimoles = 17.11mol
You mean 17.11 millimoles


You dissolve 1 gram of NaCl in 100mL of water, convert this conc to mg/mL
- 1000/100 = 10mg/mL



What volume of this solution is required in order to make 500mL of a 10microgram/mL solution?
- 10microgram/mL solution equals 0.01mg/mL
- mg/mL x mL = mg
- 0.01 x 500 = 5mg required
- mg / mg/mL = mL
- 5 / 10 = 0.5mL
bit long winded ..

you want 500 x 0.01 mg = 5 mg

solution concentration = 10mg/mL

so you need 0.5 mL


Dilute the entire 500mL of solution in 5L of water, what is the final conc of NaCl?
- 500ml contains 5mg of NaCl
- Moles of NaCl: 5x10^-3 / 58.44 = 8.56x10^-5mol
- Conc = moles / total volume
- Conc = 8.56x10^-5mol / 5.5 = 1.55x10^-5moldm-3

What is the molarity of NaCl in this solution?
1.55x10^-5moldm-3.
It looks like they are asking for concentration in micrograms per mL (or equivalent) and then moles per litre (mmol per litre)
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blossomx
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#5
Report Thread starter 7 years ago
#5
(Original post by charco)
:x
Thank you.
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