Differences between the gas syringe method and collecting gas over water method.

So what are the differences between the gas syringe method of collecting gas (hydrogen) and the collecting gas (hydrogen) over water methods. I would like to know the pros and cons of both. My teacher has told me that the gas syringe method is better, so it will be great if you guys could explain why the gas syringe method is better. Appreciate it :smile:

Thank you

Reply 1

Pigster

20

Are you measuring the volume at various times - to find rates, or measuring total volume produced - to do a mole calc.?

Reply 2

bpa8888

OP

3

Original post by Pigster

Are you measuring the volume at various times - to find rates, or measuring total volume produced - to do a mole calc.?

Measuring the total volume produced. I want to know why the gas syringe is better than the displacing water in a test tube method. Is it because the gas syringe method is more precise?

Reply 3

Pigster

20

I don't think there is any real advantage of a gas syringe compared to collection over water.

H2 is barely soluble in water, so that won't cause a problem.

The tolerance of measuring cylinders is comparable to (and sometimes better than) gas syringes, so precision isn't the issue.

Measuring cylinders (in schools) are available in a larger range of sizes, which is rather handy.

Both are easy to set up, use and read. Admittedly, gas syringes are quicker, but this shouldn't be fatal.

If you plan to do something like measuring out different masses of Mg to see how volume of H2 is produced and then do the mole calc to see how things went, it is worth doing the maths...

What mass of Mg will produce 100 cm3 of H2 (the standard size of gas syringe). You'd no doubt want a range of masses of Mg, to show a directly proportional relationship. So you'd want to produce something like 30-90 cm3 of H2. What mass of Mg would be needed to make 30 cm3? Do you have a balance able to measure than mass of Mg accurately? Or should you be scaling the whole thing up? Do you have the gas syringes available for this volume of H2? (I'll bet you have the measuring cylinders). BTW, I haven't done the maths, I'm using my (rather large) gut.

It's easier to draw.

OP

Original post by Pigster

I don't think there is any real advantage of a gas syringe compared to collection over water.

H2 is barely soluble in water, so that won't cause a problem.

The tolerance of measuring cylinders is comparable to (and sometimes better than) gas syringes, so precision isn't the issue.

Measuring cylinders (in schools) are available in a larger range of sizes, which is rather handy.

Both are easy to set up, use and read. Admittedly, gas syringes are quicker, but this shouldn't be fatal.

If you plan to do something like measuring out different masses of Mg to see how volume of H2 is produced and then do the mole calc to see how things went, it is worth doing the maths...

What mass of Mg will produce 100 cm3 of H2 (the standard size of gas syringe). You'd no doubt want a range of masses of Mg, to show a directly proportional relationship. So you'd want to produce something like 30-90 cm3 of H2. What mass of Mg would be needed to make 30 cm3? Do you have a balance able to measure than mass of Mg accurately? Or should you be scaling the whole thing up? Do you have the gas syringes available for this volume of H2? (I'll bet you have the measuring cylinders). BTW, I haven't done the maths, I'm using my (rather large) gut.

Thank you so much :smile:

)

Water is messy.

Original post by Infraspecies

Water is messy.

Nooo, water is splashy-fun!

Reply 8

Infraspecies

16

Original post by Pigster

Nooo, water is splashy-fun!

I like things to be as dry as possible, without having to perform ridiculous acts like ovening my glassware, or using a glovebox.
Quite frankly, I find hygroscopic or otherwise water-sensitive chemicals to be morally reprehensible.