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Academic Success Center HCC Brandon Campus
Stoichiometry
How can one determine the amount of product formed from a specific reactant? Or the relationship between different compounds in an equation? This is where you use stoichiometry. Take for example pizza, if 1 crust, 3 cups of cheese, and 1 cup of sauce make one pizza, how much pizza can you make with 9 cups of cheese assuming you have all other ingredients? We solve these equations with a process called dimensional analysis. Through dimensional analysis, we chemists can convert between several different units from quantity to mass to volume and many more. Here is an example of a conversion done by dimensional analysis: 9 cups of Cheese 1 pizza = 3 pizzas 3 cups of Cheese Something to note when doing dimensional analysis is that all units should cancel except for the final answer. In the context of chemistry, your dimensional analysis may look more like this: 45 g NH 3 1 mol NH 3 6.02 2 ร 10^23 molecules NH 3 3 H atoms = 4.8 ร 10^24 H atoms 1 17 g NH 3 1 mol NH 3 1 molecule NH 3 Formula Unit Atoms Molecules Mass Mole
6. 022 ร 1023
6. 022 ร 1023
Academic Success Center HCC Brandon Campus Now when doing Stoichiometry, we almost always convert to and from the unit Mole, the base unit amount of a substance. Letโs look at some conversions! Moles/Mass When using the mass of any element or compound in dimensional analysis, you will convert using that substanceโs molar mass. This is found from the atomic mass on the periodic table. For example, the molar mass of carbon is 12.0111 g/mol. Knowing this information, one can find the mass in grams of carbon when given an amount in moles or vise versa. Ex. How many grams of carbon are found in .726 moles of carbon? .726 mol C 12.0111 g C = 8.72 g C 1 1 mol C
- How many grams of Calcium are in 8.743 mole of Calcium?
- If you have 6.37 grams of Chlorine, how many moles are in this sample?
Academic Success Center HCC Brandon Campus Mass/Molecule When going back and forth from grams to number of molecules, one has to establish the value for moles of that substance as a segue and cancel the moles unit in the dimensional analysis. See this in the example below: Ex. How many molecules of NaBH 4 are in a sample of 197 grams? 197 g NaBH 4 1 mol NaBH (^4) 6.022 ๏ด 1023 molecules NaBH 4
molecules NaBH 4 1 37.83 g NaBH 4 1 mol NaBH 4
- How many molecules of CuSO 3 are in 5.7 grams of the compound?
- What is the mass of 9.64 ๏ด 1023 molecules of SeO 2?
Academic Success Center HCC Brandon Campus Stoichiometry between Molecules The essence of stoichiometry is to determine the relationships between different molecules in a reaction. Now we have to cancel between molecule specific units, i.e. grams of CO 2 (g CO 2 ) cannot cancel with grams of O 2 (g O 2 ). To do this you first convert to moles, the use the mole ratio , and convert to what you need to find. Ex. Consider the following reaction. How many moles of oxygen are required to produce 2.33 moles of water? Assume that there is excess C3H7SH present. C3H7SH(l) + 6 O2(g) โ 3 CO2(g) + SO2(g) + 4 H2O(g) 2.33 mol H 2 O 6 mol O 2 = 3.50 mol O 2 1 4 mol H 2 O
- How many moles of oxygen are formed when 58.6 g of KNO 3 decomposes according to the following reaction? The molar mass of KNO 3 is 101.11 g/mol. 4 KNO 3 (s) โ 2 K2O(s) + 2 N 2 (g) + 5 O 2 (g)
- How many moles of nitrogen are formed when 58.6 g of KNO 3 decomposes according to the following reaction? The molar mass of KNO 3 is 101.11 g/mol. 4 KNO 3 (s) โ 2 K 2 O(s) + 2 N 2 (g) + 5 O 2 (g)
Academic Success Center HCC Brandon Campus Concentration Stoichiometry Molarity (mol/L) of a substance can also be used as a method of conversion to find the mass, mole, volume and a variety of other quantities of a substance. Ex. What volume of 0.244 M KCl solution is required to react exactly with 50.0 mL of 0.210 M Pb(NO 3 ) 2 solution? 2 KCl(aq) + Pb(NO 3 ) 2 (aq) โ PbCl 2 (s) + 2 KNO 3 (aq) 50.0 mL Pb(NO 3 ) 2
1 L
Pb(NO 3 ) 2 0.210 mol Pb(NO 3 ) 2 2 mol KCl 1 L KCl = 0.0861 L or 86.1 mL 1 1000 mL Pb(NO 3 ) 2
1 L
Pb(NO 3 ) 2 1 mol Pb(NO 3 ) 2 0.244 mol KCl
- What volume of 0.305 M AgNO 3 is required to react exactly with 155.0 mL of 0.274 M Na 2 SO 4 solution? 2 AgNO 3 (aq) + Na 2 SO 4 (aq) โ Ag 2 S(aq) + Na 2 SO 4 (aq)
- According to the following reaction, what mass of PbCl 2 can form from 235 mL of 0.110 M KCl solution? Assume that there is excess Pb(NO 3 ) 2. 2 KCl(aq) + Pb(NO 3 ) 2 (aq) โ PbCl 2 (s) + 2 KNO 3 (aq)
Academic Success Center HCC Brandon Campus Answer Key
- 350.4 g
- 0.180 mol
- 1.863 ๏ด 1024 molecules
- 3.16 mol
- 2.4 ๏ด 1022 molecules
- 178 g
- 0.724 mol
- 0.290 mol
- 99.8 g
- 10.9 g
- 0.198 g
- 17.01 g
- 278 mL
- 3.59 g
