The mole road map, mole ratios, limiting reagents and percent yield with worked examples: practice questions and flashcards. Free, no sign-up.
Subject: chemistry
A chemist needs to measure out a specific number of molecules for a reaction. Given that individual atoms are extraordinarily light, what is the most practical reason for using the mole concept instead of attempting to count individual particles?
Answer: One mole of any substance has a mass in grams numerically equal to its relative atomic or molecular mass, enabling measurement by weight.
The mole concept provides a bridge between the atomic scale and laboratory measurements. Because one mole of a substance has a mass in grams numerically equal to its relative atomic or molecular mass, chemists can use a balance to weigh out a specific number of particles (in moles) without having to count them individually.
What is the molar mass of ammonium sulfate, (NH₄)₂SO₄, given the approximate atomic masses: N=14.0, H=1.0, S=32.1, O=16.0?
Answer: 132.1 g/mol
To calculate the molar mass of (NH₄)₂SO₄, you must account for the subscript '2' outside the parentheses, which multiplies everything inside. So, there are 2 nitrogen atoms, 8 hydrogen atoms, 1 sulfur atom, and 4 oxygen atoms. The calculation is 2(14.0) + 8(1.0) + 32.1 + 4(16.0) = 28.0 + 8.0 + 32.1 + 64.0 = 132.1 g/mol.
Why is the mole considered the 'central hub' for stoichiometry calculations?
Answer: It is the only unit that can be directly converted to grams, particle counts, and gas volumes.
The mole is the central hub because grams, particle counts, and gas volumes cannot be compared directly with each other. Instead, each of these quantities must be converted to or from moles, making the mole the essential intermediate step in all stoichiometry calculations.
In the reaction N₂ + 3H₂ → 2NH₃, if you start with 28.0 g of nitrogen (N₂) and 3.0 g of hydrogen (H₂), which reactant is the limiting reagent? (Approximate molar masses: N₂=28.0 g/mol, H₂=2.0 g/mol)
Answer: Hydrogen (H₂) because it is insufficient relative to the mole ratio demanded by nitrogen.
First, convert masses to moles: 28.0 g N₂ ÷ 28.0 g/mol = 1.00 mol N₂. And 3.0 g H₂ ÷ 2.0 g/mol = 1.5 mol H₂. The balanced equation requires 3 moles of H₂ for every 1 mole of N₂. If 1.00 mol N₂ is present, it would require 3.0 mol H₂. Since only 1.5 mol H₂ is available, hydrogen is the limiting reagent because it runs out first.
Why is it critically important to balance a chemical equation before performing any stoichiometry calculations?
Answer: The coefficients in a balanced equation provide the exact mole ratios necessary for converting between substances.
Balancing a chemical equation ensures that the coefficients accurately represent the mole ratios between reactants and products. These mole ratios are fundamental for all stoichiometry calculations, allowing chemists to correctly convert between moles of different substances in a reaction. Without a balanced equation, these ratios are wrong, leading to invalid calculations.
Chemists cannot count atoms one by one, so they count them in bulk using the {0}.
Answer: mole
The mole is a unit used by chemists to count particles in bulk, as individual atoms are too small to count.
One mole is 6.022 × 10²³ particles, a value called {0} after the Italian scientist Amedeo Avogadro.
Answer: Avogadro's number
Avogadro's number defines the quantity of particles in one mole.
The {0} of a substance is the mass of one mole of it, in grams per mole (g/mol).
Answer: molar mass
Molar mass is the mass of one mole of a substance, expressed in grams per mole.
For an element, the molar mass is the {0} from the periodic table expressed in g/mol.
Answer: atomic mass
The relative atomic mass, often referred to as atomic mass in this context, from the periodic table is used to determine the molar mass of an element.
Water has a relative {0} of 18.0, so one mole of water has a mass of 18.0 g.
Answer: molecular mass
The relative molecular mass is used to determine the molar mass of a compound.
The coefficients of a balanced chemical equation state the {0}, not the ratio of grams.
Answer: mole ratio
Coefficients in a balanced equation represent the mole ratio between reactants and products.
The {0} is the reactant that runs out first; it stops the reaction and determines how much product can form.
Answer: limiting reagent
The limiting reagent is the reactant that is completely consumed, thereby limiting the amount of product formed.
The amount of product calculated from stoichiometry is the {0}, the maximum possible if every particle of limiting reagent reacts.
Answer: theoretical yield
Theoretical yield is the maximum amount of product that can be produced based on stoichiometric calculations.
The amount actually obtained in the experiment is the {0}, and it is measured on a balance.
Answer: actual yield
Actual yield is the amount of product physically obtained from an experiment.
The actual and theoretical yields are compared with {0} = (actual yield ÷ theoretical yield) × 100.
Answer: percent yield
Percent yield is a measure of the efficiency of a reaction, comparing actual yield to theoretical yield.
At {0} (defined in most school courses as 0 °C and 1 atm), one mole of any ideal gas occupies 22.4 liters.
Answer: STP
STP stands for Standard Temperature and Pressure, conditions under which one mole of an ideal gas occupies 22.4 liters.
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