[strewart]

This is messing me around a bit and I seem to be finding two different techniques of converting SG to molarity... I am very familiar with normal chemistry calculations but haven't come across SG in any detail before.

For an example, SG=1.2 (assume 20C)
1.
SG 1.2 means 1200g/L
Water density ~1000g/L
Therefore there is 200g/L of acid (1200-1000)
n=m/M so 200/98.1 = 2.04 M

2.
I found conversion tables saying 1.2 SG ~28%wt (20C)
so 1200g/L * 0.28 = 336g of acid in a litre
336g/L / 98.1 = 3.43 M

Which is correct? I am leaning towards the second one.

And a further question of this, I have x volume of a solution and after a reaction know what mass of sulphuric acid was consumed and the SG at start and end (volume stayed the same), to calculate volume I can do Δm/ΔC.

Can I use ΔSG on the bottom line? Or if 2. above is correct do I need to use the concentration (g/L) calculated from SG*%wt? I am again assuming I need to calculate g/L first from the table, but I'm second guessing myself because the result using ΔSG is a lot closer to what I expected.

[Hunter2]

The second one is correct. In the first one you mix specific gravity and the  volume.

[Enthalpy]

[...]
SG 1.2 means 1200g/L
Water density ~1000g/L
Therefore there is 200g/L of acid (1200-1000)
[...]

The acid brings volume as well! Implicitly, you added its mass only.
And what is worse: volumes don't always add, especially if "mixing" produces a strong temperature change.

[strewart]

I have seen a fair few websites that suggest the first method... Is it just a simplification for industry 'bucket' chemistry where being exact isn't always necessary? Intuitively if SG = d(substance) / d(water) and the density of water is 1 then it should work, but I can certainly see that chemical interactions and temperature would influence it.

Is it possible to work out a concentration from SG without looking up a table? Using a ratio of water density and sulphuric acid density (1.84 at 98%) somehow? Or do you need more information than just SG of a given dilution?

[Hunter2]

You need either concentration, Percentage or specific gravity. Then you can calculate the rest. Some times the mixing equation or mixing cross work as well, especially calculate only in mass and not volume. For volume the specific gravity is a must.

[Enthalpy]

I have seen a fair few websites that suggest the first method... Is it just a simplification for industry 'bucket' chemistry where being exact isn't always necessary? Intuitively if SG = d(substance) / d(water) and the density of water is 1 then it should work, but I can certainly see that chemical interactions and temperature would influence it.

The first attempts isn't a method, it's an error. The acid's volume won't vanish. Interactions and thermal expansion bring corrections (sometimes important ones) to the sum of the volumes.