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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.

2. (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.

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..
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.
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)
5. (Original post by charco)
:x
Thank you.

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