Need Help badly...help me please.. Need a chemist!!?

Question

The following are possible sources of experimental errors. State the effect on the indicated variable as high result, low result or no effect and explain your answer.

a) The volume of cold air - droplets of water in flask when heated by the boiling water.

b) The volume of cold air - the glass tubing tip not sealed off as the hot flask is transferred to the bucket of water.

c) The volume of cold air - as the flask is cooled down, the glass tubing tip is sealed off but moved up and down the water level in the basin.

d) The molar volume of hydrogen gas - the magnesium ribbon is not completely reacted when the volume of the hydrogen gas is taken.

e) The experimental volume of hydrogen gas - the 10 ml graduated cylinder is not completely covered with liquid when the stopper containing the magnesium ribbon is inserted to it.

f) Experimental molar volume of hydrogen gas - the liquid level inside the 10 ml graduated cylinder is higher than that of outside when the volume of hydrogen gas is taken.

g) Empirical value of ideal gas constant - the experimental value of molar volume of hydrogen gas at STP is high.

h) Experimental value of molecular weight of HCL - the distances traveled by both gases is not measured immediately, and the white ring moved towards NH3.

i) Experimental molar volume of hydrogen gas - the experiment is performed in Baguio City, where the atmospheric pressure is 737 mmHg.

Answer 1

Your best bet on these homework problems is to carefully consider what is going on in each experiment.  If you understand what's going on, then it's easy to predict what happens when you change the conditions (or add sources of error).  Detailed help is difficult on these, since most of them seem to depend on some context which you didn't provide.

Taking (d) and (e) as an example, and assuming that you're measuring the volume of hydrogen gas evolved by burning magnesium ribbon in water:

Magnesium burns.  This consumes oxygen from the water, leaving hydrogen behind.  The hydrogen collects in the graduated cylinder.  If you know the number of moles of magnesium burned, you can calculate the number of moles of hydrogen produced.  If you measure the volume of hydrogen produced, then you can use the volume and the number of moles to calculate the molar volume (that's the volume per mole, so if 0.02 moles takes up 1.4 mL, then the molar volume is 0.07 L/mol).

Now, what happens if you don't burn _all_ of the magnesium ribbon?

What happens if you don't collect _all_ of the hydrogen?

If it makes it easier, try plugging in some numbers - they don't even have to be real.  What happens if you think you're burning 1.0 g of magnesium ribbon and you only burn 0.9 g of it?  Calculate the final answer both ways - is it higher or lower as a result of the error?

What happens if you expected to collect 5 mL but you only collected 3 mL because of (3)?

So the real solution to these problems - or rather the steps to solve them - is this:

1) Understand what's going on.  (Ask us about that, if you want.  It's more fun to explain a process than to just do someone else's homework.)

2) Think through the changes they ask about.  When you understand the situation, changes are usually pretty easy to explain.