Chemistry 101
Homework 5
Assigned – June 28th
Due – July 5th
Reminder No Class July 3th
Instructions: Answer each of the following questions. Use the proper number of
significant figures whenever necessary.
1. If 5.00g of lithium oxide are combined with 5.00g of carbon dioxide what is the
most lithium carbonate that can be made. How much of the excess reagent is left
over?
Li2O( s ) + CO2( g ) → Li2CO3( s )
5.00 gLi2O 1moleLi2O 1moleLi2CO3
73.88 g
×
×
×
= 128 g
29.88 g
1moleLi2O 1moleLi2CO3
5.00 gCO2 1moleCO2 1moleLi2CO3
73.88 g
×
×
×
= 8.40 g
44.00 g
1moleCO2 1moleLi2CO3
The most that can be made is 8.40g of lithium carbonate.
2. 2.85g of lead(II)nitrate is being added to excess sodium chromate to make
lead(II)chromate and sodium nitrate. In the end, 1.62g of lead(II)chromate is
obtained. What is the actual and theoretical yield of lead(II)chromate.
Pb( NO3 ) 2( aq ) + Na2CrO4 → PbCrO4( s ) + 2 NaNO3( aq )
2.85 gPb( NO3 ) 2 1molePb( NO3 ) 2 1molePbCrO4
323.19 g
×
×
×
= 2.78 g
331.2 g
1molePb( NO3 ) 2 1molePbCrO4
1.62 g
*100 = 58.3%
2.78 g
3. In the lab, it is not unheard of to get greater than 100% yield. Describe some
reasons for obtaining a better than 100% yield.
There are a lot of reasons. Some things to check would be calculations, including
formula weights. If they are not correct, then yields will not be. If they are
correct then you got more mass in the end than expected. This could be from
solvent that may have been involved, as well as mass of other contaminants as
well. The problem could be too much mass.
4. Cinnamic alcohol has the formula C9H10O. What is the mass percentage of every
element in the compound? How many molecules are in a 0.469g sample of
cinnamic alcohol? How many atoms of carbon are in a 0.469g sample of
cinnamic alcohol?
C9 H10O = 134 g
mole
9*12
*100 = 80.6%
134
1*10
%H =
*100 = 7.46%
134
1*16
%O =
*100 = 11.9%
134
0.469 g 1moleC9 H10O 6.022 x1023 molecules
×
×
= 2.11x1021 molecules
134 g
1mole
%C =
0.469 g 1moleC9 H10O 6.022 x1023 molecules 9atomsC
×
×
×
= 1.90 x1022 atomsC
134 g
1mole
1molecule
5. Ozone in the atmosphere is said to react with nitrogen monoxide according to the
following reaction.
O3(g) + NO(g) → O2(g) + NO2(g)
If 0.740g of ozone reacts with 0.670g of NO, how many grams of NO2 can be
made? What is the limiting reactant?
0.740 gO3 1moleO3 1moleNO2
46.0 g
×
×
×
= 0.709 g
48.0 g
1moleO3 1moleNO2
0.670 gNO 1moleNO 1moleNO2
46.0 g
×
×
×
= 1.03 g
30.0 g
1moleNO 1moleNO2
The theoretical yield of NO 2 is 0.709g and the limiting reagent is ozone.
6. In the chemical reaction
N2 + H2 → NH3
We compare amounts in terms of moles as opposed to grams, in order to make
predictions. Why?
When balanced, the equation above represents a number of molecules or particles
coming together and the ratios in which they combine. Gram amounts do not
necessarily relate to the number of particles equally. That is 1.00g of nitrogen
has fewer particles than 1.00g of hydrogen. It is therefore helpful to compare
things in moles, which represents the number of particles.
7. Looking ahead to gases. Give a definition of pressure. What is it, and how do
you experience it on a day to day basis?
Pressure is defined as a force applied per unit area. It is the force exerted by the
weight of a column of air above us at all times. At higher elevations, there is less
air above and so pressure decreases with increasing altitude. We are familiar
with tire pressure (32 psi) which is maintained by controlling the amount of air
inside a constant volume tire.