Download Stoichiometry Workbook - Concentration

Stoichiometry Workbook 2
Mixture
Solute : The compound
that is dissolved in the
mixture
Solvent : The substance
that the solute is
dissolved in.
𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦 =
𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒
𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑖𝑛 𝑑𝑚3
Solve the problems below:
1. What is the molarity of a solution in which 58 g of NaCl are dissolved in 1.0 dm3 of solution?
Process: It is required to find out the number of moles of the solute that are dissolved in 1 .0 dm3 of
solution. For this reason we have to convert the mass of solute given to us in moles:
(𝑀𝑟(𝑁𝑎𝐶𝑙)=58.5 𝑔/𝑚𝑜𝑙 therefore
58.5g of NaCl (s)
correlate to 1 mole of NaCl
58 g of NaCl (s)
correlate to x mole of NaCl
𝑥=
58 𝑔 𝑥 1 𝑚𝑜𝑙
58.5 𝑔
= 0.99 mol
Since we have 0.99 mol dissolved in 1.0 dm3 of solution, the solution has a concentration (or molarity)of
0.99 M.
(***The unit is pronounced as Molars – NOT to be confused with moles! 1 molar = 1M = 1 mol/dm3)
2. What is the molarity of a solution in which 10.0 g of AgNO3 is dissolved in 500 cm3 of solution?
3. How many grams of KNO3 should be dissolved in 2.00 dm3 of solution in order to prepare a
0.500M solution?
4. To what volume should 5.0g of KCl be diluted to in order to prepare a 0.25 M solution?
5. How many grams of CuSO4•5H2O are needed to prepare a 100 cm3 of an aqueous CuSO4
solution with concentration of 0.10M?
(Advanced question)
Composition of hydrates:
Hydrates are ionic compounds with water molecules mechanically traped within their crystal latice. The
water is in a specific ratio to each formula unit of the salt. For example, the molecular formula for coper
sulfate pentahydrate: CuSO4•5H2O, indicates that there are 5 water molecules present for each one
formula unit of CuSO4.
Answer the following questions:
1. What percentage of water is found in CuSO4•5H2O?
Solution: The hydrated salt hasa total molecular mass (including water) of 249.7 g/mol
The mass of the water present is 90.0 g/mol (5 x 18 g/mol)
The percentage of water present is:
I
In 249.7 g/mol of salt there are 90 g/mol of water present
f 100.0 g/mol of salt are assumed then X g/mol of water present
𝑥=
100.0 𝑥 90.0
249.7
= 36.1 g/mol
A 36.1 % of water is present in the molecule.
2. What percentage of water is found in Na2S•9H2O?
3. A 5.0 g of a hydrate of BaCl2 was heated and only 4.3 g of the anhydrous salt remained. What
percantage of water was in the hydrate?
4. A 2.5g sample of a hydrate of Ca(NO3)2 was heated and only 1.7 g of the anhydrated salt
remained. What percentage of water was in the hydrate?
5. A 3.0 g sample of Na2CO3•H2O is heated to constant mass. How much anhydrous salt remains?
6. A 5.0 g sample of Cu(NO3)2•xH2O is heated to consatnt mass and only 3.9g of the anhydrate salt
reamins. What is the value of x?
Mole-Mole Problems
1. N2 (g)+ 3H2 (g) →2NH3 (g)
How many moles of hydrogen are needed to completely react with two moles of nitrogen?
2. 2KClO3(s) → 2KCl (s) + 3O2 (g)
How many moles of oxygen are produced by the decompostition of six moles of potassium
chlorate?
3. Zn (s) + HCl (aq) → ZnCl2 (s) + H2 (g)
How many moles of hydrogen are produced from the reaction of three moles of zing with an
excess of hydrochloric acid?
4. C3H8(g) + 5O2(g) → 3 CO2(g) + 4 H2O(l)
How many moles of oxygen are necessary to react completely with 4 moles of propane (C3H8)
5. K3PO4 (aq) + Al(NO3)3(aq) →3KNO3 (aq) + AlPO4(aq)
How many moles of potassium nitrate are produced when two moles of potassium phosphate
react with two moles of aluminium nitrate?