[canadian_mike]

ok, i did a experiemnt in class and am now doing a lab report. i found my PY to be 100%. i have to explain y in the discussion. i have reasons for a lower PY and a higher PY but have had no luck in finding anything for a PY that is 100.

the experiment was to put 5 g of CuCl2 in a beaker with 50ml of water. then place 2 g of iron filings in. let the reaction happen then decant the solution. eventually the product remaining would be clean and u would put a little water just to cover it the let it sit for 3 days. the mass would be taken again whe we got back on monday.

ive found the following reasons y your PY could be lower:
-decanting error(losing sample)
-reaction doesnt fully happen because the 2 elements on the reactivity series r cose together

PY could be higher:
-rust weighs more than oxygen
-copper carbonate hydroxide present
-cucl2 excessreactant, wash it off, let it evaporate therefore unreacted chloride remaining.

any help please.

[Borek]

What about the idea that positive and negative errors canceled out? Or measurement error?

You haven't posted any information about the precision of your measurements - how large error can you miss? How much CuCl2 have you weighted - 5g? 5.00g? What was the iron filling mass increase? 5g? 5.00g? In the first case you will miss 10% error, in the second - 0.1% max. Additionally, copper chloride is hydrated and I have no idea how constant is the composition - many hydrated salts can have changing amount of water depending on the conditions theye are kept in.

Just some thoughts