General

(These are all for number one)
Reason 1. Equilibrium points. Although I can write a balanced equation: 2N+02->2NO. This does not happen because the equilibrium constant prefers 2N and 02 rather than 2NO. In your case, although most of the reactants will shift into products the equilibrium constant won't let it go into 100% yield.
Reason 2. Some might have evaporated (?). I'm not quite sure but if they are dissolved in water then it is probable.
Reason 3. Rounding error.

(Number 2)
Option 1. Have one of the reactants concentration greatly surpasses the other reactant. This will make the yield closer to 100%
Option 2. Using a lid to protect it from evaporating and losing reactants.
I don't know anymore... Chemistry is not my stronghold.

[quote=aznseal]The yield will be less because the reaction only goes on for as long as their is the limiting reagent present. Because the limiting reagent is impure, there will be "less of it to react" causing the reaction to stop sooner causing a less actual yield.

And probably what causes % differences are human error, temperature difference, impurities, missed measurements, etc[/quote]

OMG dude thanks soooooooooooooooooooooooooooooo much
ur the best

1. This all depends on the experiment you preformed. 100% yield only occurs under ideal conditions which is rare, so you have to look at the experiment you performed, the procedure and find areas in which errors could have resulted. Ex. Say you had to heat a hydrate for a long period of time in order to evaporate the water within the compound and measure its mass. If not heated properly, some of the water may still be retained by the compound, the measured mass would be skewed, and any subsequent calculations (such as percent yield) would be subject to the inaccuracy of your techniques.
2. You have to look at your experiment. :x
3. The percent yield would be lower than expected, because you would have less of the reacting compound and would only be able to produce so much product.

The yield will be less because the reaction only goes on for as long as their is the limiting reagent present. Because the limiting reagent is impure, there will be "less of it to react" causing the reaction to stop sooner causing a less actual yield.

And probably what causes % differences are human error, temperature difference, impurities, missed measurements, etc

[quote=aznseal]1. The conditions are rarely (if ever) optimal enough to get 100% of the theoretical yield.
2. You should be able to do this.
3. It will lower the % yield because less will have reacted[/quote]

YEAHH ur on, i love u dude u helped me out alot last time too
but can u explain why the % yeild will be less
and for number 1, what should i write for the 3 reasons,

1. The conditions are rarely (if ever) optimal enough to get 100% of the theoretical yield.
2. You should be able to do this.
3. It will lower the % yield because less will have reacted