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Chapter 3: Problem 17

According to the law of conservation of mass, mass cannot be gained or destroyed in a chemical reaction. Why can't you simply add the masses of two reactants to determine the total mass of product?

Short Answer

Expert verified
The law of conservation of mass states that mass cannot be created or destroyed in a chemical reaction. To calculate the total mass of products in a reaction, a balanced chemical equation is needed to ensure the same number of atoms for each element on both sides. Simply adding the masses of reactants without considering the balanced equation would violate this law, as it would wrongly assume a 1:1 ratio of atoms. Instead, the balanced equation allows us to find the correct ratio of reactants and products, ensuring that the law of conservation of mass is obeyed.

Step by step solution

01

Introduction to the Law of Conservation of Mass

The law of conservation of mass states that mass cannot be created or destroyed in a chemical reaction. This means that the total mass of the reactants before a reaction occurs must be equal to the total mass of the products after the reaction is completed.
02

Establishing the Importance of Balanced Chemical Equations

In order to calculate the total mass of products in a chemical reaction, it is essential to have a balanced chemical equation. A balanced equation ensures that the same number of atoms for each element is present on both sides of the equation. This is crucial because atoms are neither created nor destroyed during a chemical reaction. Balancing a chemical equation helps in determining the correct ratio of reactants and products.
03

Demonstrating with an Example

Let's consider a chemical reaction between sodium (Na) and chlorine (Cl) to form sodium chloride (NaCl). Unbalanced equation: \(Na + Cl \rightarrow NaCl\) In the unbalanced equation, we cannot simply add the masses of sodium and chlorine to determine the total mass of sodium chloride because a 1:1 ratio of sodium and chlorine atoms is assumed. To balance the equation, we must ensure the number of atoms for each element is equal on both sides: Balanced equation: \(2Na + Cl_2 \rightarrow 2NaCl\) Now, we can see that 2 moles of sodium react with 1 mole of chlorine to produce 2 moles of sodium chloride. So, if we want to determine the total mass of the product, we need to multiply the moles of each product by its respective molar mass, and then sum the masses: Mass of 2 moles of NaCl = (2 moles of Na x molar mass of Na) + (1 mole of Cl x molar mass of Cl) = (2 x 22.99 g/mol) + (1 x 70.90 g/mol) = 45.98 g + 70.90 g = 116.88 g. Hence, we can't simply add the masses of reactants without considering the balanced chemical equation, which ensures that the law of conservation of mass is obeyed.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Balanced Chemical Equations
Understanding the essence of balanced chemical equations is crucial when diving into the complex world of chemical reactions. Think of a chemical equation as a recipe for a dish; it needs the exact amount of each ingredient to turn out just right. Similarly, in chemistry, a balanced chemical equation ensures that the number of atoms for each element is the same on both sides of the arrow. This balancing act is not just for aesthetics or academic rigor; it's a representation of the conservation of mass.

When we balance an equation, we show respect for this fundamental law of chemistry. This law tells us that atoms aren't lost or gained during a chemical reaction; they're merely rearranged. If we fail to balance the equation, our calculations for reactants and products will be off, leading to an erroneous understanding of the reaction. So, in essence, balanced equations serve as the bedrock upon which accurate predictions and calculations of a chemical reaction are made.
Chemical Reaction Stoichiometry
Now let's turn our focus to chemical reaction stoichiometry. Imagine you're the conductor of an orchestra where each musician, or in our case, each atom or molecule, has to play its part at precisely the right moment. Stoichiometry helps us to understand this exact dynamic in chemical reactions. It is the study of the quantitative relationships, or ratios, of reactants and products in a chemical reaction.

Why Stoichiometry Matters

Stoichiometry isn't just about numbers; it's the roadmap that guides us through the conversion of reactants to products. It's essential because it tells us the proportion in which various elements combine and how much of each substance will be involved in a reaction. Mathematic formulae derived from a balanced chemical equation allow us to maintain the integrity of the conservation of mass. Without proper stoichiometry, we might as well be guessing how much of any substance will be produced or consumed in a reaction.
Molar Mass Calculation
To practically apply the law of conservation of mass and stoichiometry, we need the skill of molar mass calculation. Remember that molar mass is the weight of one mole of a substance, usually expressed in grams per mole (g/mol). It acts as a bridge between the microscopic world of atoms and molecules and the macroscopic world we can measure.

By knowing the molar mass, we can convert the weight of a substance to the number of moles and vice versa. This conversion is fundamental because chemical reactions are quantified and compared in moles, not direct mass. Think of it like converting currency when you travel; you need to know the conversion rate to make sense of prices in a foreign country. In chemistry, molar mass provides us with that conversion rate, so we can 'translate' the mass of a substance into an amount (moles) that's useful for stoichiometric calculations.

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Most popular questions from this chapter

Chloral hydrate \(\left(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{Cl}_{3} \mathrm{O}_{2}\right)\) is a drug formerly used as a sedative and hypnotic. It is the compound used to make "Mickey Finns" in detective stories. a. Calculate the molar mass of chloral hydrate. b. What amount (moles) of \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{Cl}_{3} \mathrm{O}_{2}\) molecules are in \(500.0 \mathrm{~g}\) chloral hydrate? c. What is the mass in grams of \(2.0 \times 10^{-2}\) mole of chloral hydrate? d. What number of chlorine atoms are in \(5.0 \mathrm{~g}\) chloral hydrate? e. What mass of chloral hydrate would contain \(1.0 \mathrm{~g} \mathrm{Cl}\) ? f. What is the mass of exactly 500 molecules of chloral hydrate?

Glass is a mixture of several compounds, but a major constituent of most glass is calcium silicate, \(\mathrm{CaSiO}_{3-}\) Glass can be etched by treatment with hydrofluoric acid; HF attacks the calcium silicate of the glass, producing gaseous and water-soluble products (which can be removed by washing the glass). For example, the volumetric glassware in chemistry laboratories is often graduated by using this process. Balance the following equation for the reaction of hydrofluoric acid with calcium silicate. $$ \mathrm{CaSiO}_{3}(s)+\mathrm{HF}(a q) \longrightarrow \mathrm{CaF}_{2}(a q)+\mathrm{SiF}_{4}(g)+\mathrm{H}_{2} \mathrm{O}(l) $$

With the advent of techniques such as scanning tunneling microscopy, it is now possible to "write" with individual atoms by manipulating and arranging atoms on an atomic surface. a. If an image is prepared by manipulating iron atoms and their total mass is \(1.05 \times 10^{-20} \mathrm{~g}\), what number of iron atoms were used? b. If the image is prepared on a platinum surface that is exactly 20 platinum atoms high and 14 platinum atoms wide, what is the mass (grams) of the atomic surface? c. If the atomic surface were changed to ruthenium atoms and the same surface mass as determined in part b is used, what number of ruthenium atoms is needed to construct the surface?

A gas contains a mixture of \(\mathrm{NH}_{3}(g)\) and \(\mathrm{N}_{2} \mathrm{H}_{4}(\mathrm{~g})\), both of which react with \(\mathrm{O}_{2}(g)\) to form \(\mathrm{NO}_{2}(g)\) and \(\mathrm{H}_{2} \mathrm{O}(g)\). The gaseous mixture (with an initial mass of \(61.00 \mathrm{~g}\) ) is reacted with \(10.00\) moles \(\mathrm{O}_{2}\), and after the reaction is complete, \(4.062\) moles of \(\mathrm{O}_{2}\) remains. Calculate the mass percent of \(\mathrm{N}_{2} \mathrm{H}_{4}(\mathrm{~g})\) in the original gaseous mixture.

ABS plastic is a tough, hard plastic used in applications requiring shock resistance. The polymer consists of three monomer units: acrylonitrile \(\left(\mathrm{C}_{3} \mathrm{H}_{3} \mathrm{~N}\right)\), butadiene \(\left(\mathrm{C}_{4} \mathrm{H}_{6}\right)\), and styrene \(\left(\mathrm{C}_{8} \mathrm{H}_{8}\right)\). a. A sample of ABS plastic contains \(8.80 \% \mathrm{~N}\) by mass. It took \(0.605 \mathrm{~g}\) of \(\mathrm{Br}_{2}\) to react completely with a \(1.20-\mathrm{g}\) sample of ABS plastic. Bromine reacts \(1: 1\) (by moles) with the butadiene molecules in the polymer and nothing else. What is the percent by mass of acrylonitrile and butadiene in this polymer? b. What are the relative numbers of each of the monomer units in this polymer?
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