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## Key Questions

Consider an exothermic combustion reaction. The amount of heat may be treated as a stoichiometric product, precisely dependent on the amount of hydrocarbon combusted.

## Explanation:

Methane combustion drives our civilization to a very great degree:

#CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O, DeltaH = -890# #kJ# #mol^-1# .

The quoted enthalpy of combustion is per mole of reaction as written. You don't have to know these; you do have to know how to balance the equation. Because this energy is associated with the combustion of 1 mol of methane, I could also treat the evolved energy as a reagent or product in the reaction:

i.e. #CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O + 890# #kJ# .

It would be on the reactant side if the reaction was endothermic. In other words the minus sign denotes evolution of heat. Alternatively, #890# #kJ# of heat are evolved from the above reaction, so I am treating energy as a product just as carbon dioxide and water are (as indeed it is; of course the heat is a consequence of the formation of water and carbon dioxide bonds). If less than 16 g (1 mol) methane are combusted, the heat evolved will diminish stoichiometrically. Does this address your question?

## Heats of Reaction - Hess' Law

This activity provides a demonstration of Hess' Law using three reactions: the solubility NaOH in water, the solubility NaOH in HCl and the reaction of a solution of HCl and a solution of NaOH.

## Chemical Potential- Staircase Demonstration

As we look for connection points across disciplines, we are increasingly drawn to chemical potential as the connection point. This simulation shows particles thermally distributed on a staircase. A linear increase…

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## High School Chemistry : Stoichiometry

Study concepts, example questions & explanations for high school chemistry, all high school chemistry resources, example questions, example question #1 : stoichiometry.

Consider the following reaction:

Since we are given the mass of nitrogen gas that will be used, we will need to convert the amount into moles. We can then compare the molar ratios in the balanced reaction, and multiply the moles of ammonia by its molar mass. Using dimensional analysis, we can use a calculation that will allow us to end with "grams of ammonia" as the solved unit.

## Example Question #2 : Stoichiometry

What is the percent composition by mass of oxygen in a molecule of glucose?

First, we must find the molecular weight of the entire molecule of glucose. We do this individually for each element. Using oxygen as an example, we multiply the atomic mass (16) by the number of atoms per glucose molecule:

We do similar calculations for carbon and hydrogen to find our total molecular weight for glucose.

Sum these values to find the molecular weight.

Next, to find percent composition by mass for oxygen, we are being asked to find how much of one molecule of glucose's weight comes from oxygen. To do this we divide the mass of oxygen by the molecular weight:

Thus 53.3% of the mass of glucose comes from oxygen alone.

## Example Question #3 : Stoichiometry

What is the mass of 25 iron atoms?

First, convert the number of atoms to moles using Avogadro's number:

Next, convert moles of iron to grams of iron using the molar mass:

## Example Question #4 : Stoichiometry

Next, use dimensional analysis to find the number of moles in the given mass of the sample.

## Example Question #5 : Stoichiometry

Next, use dimensional analysis to convert grams to moles.

Use the correct number of significant figures.

## Example Question #6 : Stoichiometry

A silicon microchip weighs 7.2mg. How many atoms of silicon are in the microchip?

First convert the 7.2mg of silicon into grams:

Then convert grams of silicon into moles of silicon:

Finally, convert moles of silicon into atoms using Avogadro's number:

Since 7.2mg of silicon is much less than 1 mole (which we know weighs 28.09g) our answer makes sense as it is much less than Avogadro's number.

None of these

Use Avogadro's number to convert from atoms to moles:

## Example Question #8 : Stoichiometry

What is the atomic number of calcium?

The atomic number is equal to the number of protons in an atom and can be determined using the periodic table. The atomic number is located on the top left of the box containing the desired chemical symbol. The chemical symbol for calcium is Ca and the atomic number is 20.

## Example Question #9 : Stoichiometry

How many neutrons does calcium-42 have?

The 42 in calcium-42 represents the mass number of calcium. The mass number equals the sum of the number of protons and the number of neutrons. Using the periodic table, the number of protons in calcium can be determined using the atomic number. The atomic number of calcium is 20.

Using all this information the number of neutrons can be determined by subtracting the number of protons from the atomic mass number. The number of neutrons is 22.

Atomic Mass Number = Protons + Neutrons

Atomic Mass Number - Protons = Neutrons

## Example Question #10 : Stoichiometry

The mass number is equal to the number of protons plus the number of neutrons in an atom.

Protons + Neutrons = Mass Number

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#### IMAGES

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1. Introduction to Thermochemistry (Worksheet ...

Q1: Definition Define the following terms: heat work temperature exothermic endothermic First Law of Thermodynamics system surroundings kinetic energy potential energy Q2: Fill in the Blanks Heat gained (absorbed) is considered _____________; heat lost by system to surroundings is _________________.

2. PDF Thermochemical Equations and Stoichiometry Worksheet

Thermochemical Equations and Stoichiometry Worksheet Purpose: To reinforce your understanding of the stoichiometry of thermochemical equations. Answer the questions below in the spaces provided. 1. Consider the following thermochemical equation: 2ZnS (s) + 3O 2(g) 2ZnO (s) + 2SO 2(g) ΔH = -878.2 kJ (a) How much heat is released when 3.0 mol ZnS

3. 7.5: Stoichiometry Calculations Using Enthalpy

The balanced thermochemical equation relates the energy change to moles, not grams, so we first convert the amount of N 2 to moles and then use the thermochemical equation to determine the energy change: 222.4 g N 2 × 1 m o l N 2 28.00 g N 2 × − 91.8 k J 1 m o l N 2 = − 729 k J

4. Thermochemistry (Worksheet)

Q7 For the following reaction: 1/8 S 8 (s) + H 2 (g) à H 2 S (g) ∆H = -20.2 kJ Is this an exothermic or endothermic reaction? What is ∆H for the reverse reaction? What is ∆H when 3.2 mol of S 8 reacts? What is ∆H when 20.0g of S 8 reacts? Q8 For the following reaction: MgCO3 (s) → MgO(s) + CO2 (g) M g C O 3 ( s) → M g O ( s) + C O 2 ( g)

5. 5.2.1: Practice Problems- Reaction Stoichiometry

I 2 is produced by the reaction of 0.4235 mol of CuCl 2 according to the following equation: 2CuCl2 + 4KI → 2CuI + 4KCl +I2 2 CuCl 2 + 4 KI → 2 CuI + 4 KCl + I 2 . How many molecules of I 2 are produced? What mass of I 2 is produced? Answer a Answer b PROBLEM 5.2.1.3 5.2.1. 3

6. PDF Chapter 5 Thermochemistry

from the 1:1 reaction stoichiometry. •This is an exothermic process. It releases 890.4 kJ when one mole of methane, CH 4,reacts. •The enthalpy value will change if the number of moles varies from the 1:2:1:2 reaction stoichiometry.

7. PDF Thermochemistry/Practice-Thermochemical Equations and Stoichiometry

(Ans: -1.09 x 104 KJ) Dr. Gupta/Thermochemistry - Stoichiometry/Practice/Page 1 of 2 6) Answer the following two questions using the equation given below. H2 (g) + Cl2 (g) 2HCl (g) H = -184.6 KJ What is the enthalpy change associated with the formation of 5.67 mol HCl gas in the following reaction? (Ans: -523 KJ)

8. Thermochemistry with Equation Stoichiometry

What is thermochemistry with equation stoichiometry? Answer: Consider an exothermic combustion reaction. The amount of heat may be treated as a stoichiometric product, precisely dependent on the amount of hydrocarbon combusted. Explanation: Methane combustion drives our civilization to a very great degree:

9. Thermochemistry questions (practice)

Lesson 22: Thermochemistry. Thermochemistry questions. Phase diagrams. Enthalpy. Heat of formation. Hess's law and reaction enthalpy change. Gibbs free energy and spontaneity. ... Choose 1 answer: Choose 1 answer: (Choice A) At atmospheric pressure, helium can exist in all three phases, as well as a supercritical fluid phase near absolute zero. ...

10. Stoichiometry and Thermochemical Equations

Step 1: List the known quantities and plan the problem. Known mass SO 2 = 58.0 g molar mass SO 2 = 64.07 g/mol Δ H = − 198 kJ for the reaction of 2 mol SO 2 Unknown Δ H =? kJ The calculation requires two steps. The mass of SO 2 is converted to moles. Then the mol SO 2 is multiplied by the conversion factor of ( − 198 kJ 2 mol SO 2). Step 2: Solve.

11. ChemCollective: Thermochemistry

Resource Topic: Thermochemistry . Energy and Enthalpy. Autograded Virtual Labs; Coffee Problem Autograded Virtual Lab. In this activity, students use knowledge of specific heat capacity to mix together hot coffee and cold milk to create a solution of coffee at a desired temperature. In this randomized problem, each student is given…

12. PDF Stoichiometry: Problem Sheet 1

2. Given the balanced equation: 2 N2H4(l) + N2O4(l) 3 N2(g) + 4 H2O(g) A. How many moles of dinitrogen tetrahydride are required to produce 57 moles of nitrogen? B. How many moles of dinitrogen tetroxide are required to produce 57 moles of nitrogen? C. How many moles of water are produced when 57 moles of nitrogen are made? 3.

13. Stoichiometry meets Thermochemistry Worksheet

CHEM 1 Name: _______________________ Stoichiometry meets Thermochemistry Worksheet 1. In your words, describe how you would recognize an exothermic and an endothermic reaction if you felt one or you were looking at one written on paper. H - = product = exothermic H + = reactant = endothermic H -

14. Redlands Unified School District / Homepage

Enthalpy Stoichiometry Background: Thermochemistry is the study of energy and heat associated with chemical reactions. Reactions may be exothermic (release heat) or endothermic (absorb heat), The amount of heat involved in a reaction can be calculated using stoichiometry calculations if the enthalpy change (AHO) is known for the reaction.

15. Stoichiometry 1 worksheet and key

Stoichiometry Worksheet and Key . 1.65 mol KClO 3. mol KClO 3. mol O 2 = mol O 2. 3.50mol KCl = mol KClO 3 = 0.275 mol Fe = mol Fe 2 O 3 = = Discover more from: Fundamentals of Chemistry CHM 110 . Triton College. 4 Documents. Go to course. 4. Wkst Stoich. Fundamentals of Chemistry 82% (11) 3.

16. Quiz & Worksheet

Print Worksheet. 1. At STP, how much space (in liters) will 0.750 moles of argon gas occupy? 14.3 L. 16.8 L. 29.9 L. 0.033 L. 2. How many moles of solute are in a 0.50 L solution of 3.0 molarity?

17. Stoichiometry (article)

First things first: we need to balance the equation! In this case, we have 1 1 \ce {Na} Na atom and 3 3 \ce {H} H atoms on the reactant side and 2 2 \ce {Na} Na atoms and 2 2 \ce {H} H atoms on the product side. We can balance the equation by placing a 2 2 in front of \ce {NaOH} NaOH (so that there are 2 2 \text {Na} Na atoms on each side) and ...

18. Stoichiometry questions (practice)

Stoichiometry questions. One type of anaerobic respiration converts glucose ( C_6 H_ {12} O_6 C 6H 12O6) to ethanol ( C_2 H_5 OH C 2H 5OH) and carbon dioxide. If the molecular weight of glucose is 180 180 grams/mol and the molar mass of ethanol is 46 46 g/mol, how many grams of carbon dioxide are produced when 1 1 mol of glucose is digested via ...

19. Stoichiometry Worksheets

Stoichiometry Worksheets. Stoichiometry Worksheet 1. stoichiometry Worksheet 2. stoichiometry Worksheet 3. Ap Chemistry Notes: Stoichiometry. Balancing Chemical Equations Worksheets.