[mainebrewer]

Need some math assistance here.
If I have 32 oz of phosphoric acid 85% concentration, how do I calculate the amount of water to dilute the 85% to some other percentage?
I'm having a brain cramp here!

[morticaixavier]

assuming we are talking about volumetric %

85% of 32 is 27.2. (Total Volume X %pa) = volume pa)
if you want 75% you need to have

Total Volume X .75 = 27.2

so total volume = 27.2/.75

so 36.266

so add 4.266 oz water and you have 75% concentration.

[erockrph]

volume A x concentration A = volume B x concentration B

Say you want to make 10 mL of a 8.5% concentration:

10 * 8.5 = x * 85 - solve for x

(10 * 8.5)*85 = x

You would need 1 mL of your concentrated solution to make 10mL of your dilute solution.

[ynotbrusum]

You math guys!  My problem is once I have solved for X, I don't have a container with the gradients needed!

[morticaixavier]

You math guys!  My problem is once I have solved for X, I don't have a container with the gradients needed!

what's the trouble? are you trying to measure out tenths of a ml? you can probably find an appropriately graduated syringe at the drug store.

[ynotbrusum]

Just kidding - but 4.266 ounces is just going to have to be a heavy 4.25 ounces!

[morticaixavier]

Just kidding - but 4.266 ounces is just going to have to be a heavy 4.25 ounces!

ahh yes. well we do the best we can. easier if you use metric.

[kramerog]

Generally chemicals when indicated as being a certain % concentration are in weight %.  ABV is somewhat of an aberration.

Thus
mass A x conc A = mass B x conc B.

[Jimmy K]

You can measure water by weight too. 4.26 oz (weight) = 4.26 oz (volume)

[morticaixavier]

You can measure water by weight too. 4.26 oz (weight) = 4.26 oz (volume)

only at 39*f

[a10t2]

You can measure water by weight too. 4.26 oz (weight) = 4.26 oz (volume)

Only at 4°C, and only if we're talking about avoirdupois fluid ounces. At 20°C one US customary fluid ounce of water has a mass of 1.041 avoirdupois ounces.

Anyway, the problem with solutions is that mass/volume don't usually scale linearly with concentration. If it's important to get a precise result, you'll need to look up a table for that particular solute/solvent.

[ynotbrusum]

You can measure water by weight too. 4.26 oz (weight) = 4.26 oz (volume)

Only at 4°C, and only if we're talking about avoirdupois fluid ounces. At 20°C one US customary fluid ounce of water has a mass of 1.041 avoirdupois ounces.

Anyway, the problem with solutions is that mass/volume don't usually scale linearly with concentration. If it's important to get a precise result, you'll need to look up a table for that particular solute/solvent.

Yeah, what he said.

[kramerog]

I checked the CRC handbook.  I have gravity and other properties up to 80% lactic acid.  D'Oh!  Anyway the gravity at 85% should be 1.19 g/l based on interpolation of 80% lactic acid and pure lactic acid.

[Jimmy K]

You can measure water by weight too. 4.26 oz (weight) = 4.26 oz (volume)

Only at 4°C, and only if we're talking about avoirdupois fluid ounces. At 20°C one US customary fluid ounce of water has a mass of 1.041 avoirdupois ounces.

Anyway, the problem with solutions is that mass/volume don't usually scale linearly with concentration. If it's important to get a precise result, you'll need to look up a table for that particular solute/solvent.

Eh, it's an art.

[a10t2]

That's what you always say!