Quantitative Composition of Compounds PDF

Quantitative Composition of Compounds November 5, 2024 9:31 AM A Brief History of Avogadro's Number Amedeo Avogadro was a Italian lawyer and chemist who lived from 1776 to 1856. In 1811 he proposed his hypothesis (Avogadro's Hypothesis). Avogadro's Hypothesis stated: Equal volumes of gases at...

Quantitative Composition of Compounds November 5, 2024 9:31 AM A Brief History of Avogadro's Number Amedeo Avogadro was a Italian lawyer and chemist who lived from 1776 to 1856. In 1811 he proposed his hypothesis (Avogadro's Hypothesis). Avogadro's Hypothesis stated: Equal volumes of gases at the same temperature and pressure have an equal number of molecules. Avogadro's Hypothesis led to a number of breakthroughs, including relative masses, atomic mass unit's (amu), and the mole/mol. Provided they have equal volumes and are at the same temperature and pressure! A mole is a way to count particles like atoms or molecules, similar to how a dozen counts 12 things. In the case of a mole, it’s always 6.022 x 10 23 particles (a huge number!). According to Avogadro’s Hypothesis, if you have one mole of any gas, it will fill the same amount of space as one mole of any other gas, as long as both are at the same temperature and pressure. For example, one mole of oxygen gas and one mole of nitrogen gas will each take up about 22.4 liters at standard conditions. This happens because gases with the same number of particles spread out to fill the same space, no matter what type they are. Scientists wanted a way to compare the masses of different atoms, so rather than measuring the actual mass of each atom (which would be incredibility small), they looked at how heavy each atom was relative to others (relative mass). For example, they found that 1 carbon atom weighs about the same as 12 hydrogen atoms. This means that carbon is 12 times heavier than hydrogen, or a 1:12 mass ration between hydrogen and carbon. Using this information, scientists created a standard unit for measuring atomic mass called the atomic mass unit (amu). They decided that 1 amu would equal 1/12 the mass of a carbon-12 atom (the most common form of carbon), with carbon-12 as a baseline or reference point for all atomic mass measurements. Now that we have this unit, hydrogen is defined as having 1 amu, while carbon has a mass of 12 amu. When finding the amu of other elements, scientists would use special tools such as a mass spectrometer, to measure the mass of atom. For example, when they measured oxygen, they found it was about 16 times heavier than 1 amu, meaning oxygen has a relative mass of about 16 amu in total. All elements on the periodic table were measured from this ratio, collectively creating a relative atomic mass scale which could then be applied to the periodic table. When we wish to find the total mass of a compound, we just add up the atomic masses of all the atoms within that compound. For example, take HNO 3 1. Look at each atom in the compound: HNO3 has hydrogen (H), nitrogen (N), and oxygen atoms. 2. Multiply by the number of each atom: i. Hydrogen (H): There's 1 hydrogen atom, and its atomic mass is 1.01 amu. So, 1 x 1.01 = 1.01 amu. ii. Nitrogen (N): There's 1 nitrogen atom, and its atomic mass is 14.01 amu. So, 1 x 14.01 = 14.01 amu. iii. Oxygen (O): There are 3 oxygen atoms, and each has an atomic mass of 16.00 amu. So, 3 x 16.00 = 48.00 amu. 3. Add them all together: now, add up the masses for each atom: i. 1.01 + 14.01 + 48.00 = 63.02 amu Avogadro's Number [N], 6.022 x 10 23, is just a big number used to count tiny things, like atoms or molecules. This number represents the number of atoms in exactly 12 grams of carbon-12. To put it simply, a mole is just like saying "a dozen" but on a much larger scale. Is a dozen means 12 of something, a mole means 6.022 x 10 23 of something. So, 1 mole is equal to 6.022 x 1023 items, whether those items are atoms, molecules, or even people! New Section 1 Page 1 Since atoms and molecules are so incredibly small, scientists use the mole to make it easier to count them in chemical reactions and measurements. For example, 1 mole of hydrogen atoms means you have 6.022 x 10 23 hydrogen atoms. 1 mole of water molecules means you have 6.022 x 10 23 water molecules. So if you have a mole of anything - like a mole of apples - you'd have 6.022 x 1023 apples, which is an enormous amount! The Mole Since atoms are incredibly tiny and have extremely small masses, measuring them one by one is impractical. Scientists need a way to work with large numbers of atoms in a way that's easier to understand and measure. Just like in a supermarket, where it's easier to measure fruit by weight instead of counting each piece, scientists use mass to "count" atoms on a larger (macroscale) level. Imagine you're at a store and want to buy a bunch of apples. Instead of counting each apply individually, you weigh them all together. If you know the average weight of one apple, you can calculate how many apples are in your total weight. Imagine the classic jellybean guessing game, where contestants Similarly, scientists can measure the mass of a large sample of atoms and, knowing try to estimate how many jellybeans fill a jar. As you may the average mass of a single atom, calculate how many atoms are in the sample. have learned in math class, you could use the size or average So, by knowing the average mass of atoms or molecules, scientists can easily work with volume of a single jellybean to and "count" large quantities by simply weighing them. This is where units like the mole make a more accurate estimate of come in handy, they let scientists count atoms in bulk by using their total mass. how many might fit inside the jar. Estimating the number of We are familiar with units of measurement such as kilometers or grams, but different jellybeans in a jar is similar to how disciplines and fields have units of measurements of their own. scientists estimate the number of Paper is measured by the ream (500 sheets) atoms in a sample. If you know the size of one jellybean, you can Pencils are measured by the gross (144 pencils) roughly calculate how many fit in the jar. Likewise, knowing the In chemistry, the mole (6.022 x 1023), otherwise known as Avogadro's number, is the mass of one atom helps scientists standard unit for measuring vast quantities of tiny particles, like atoms or molecules. Since estimate the total number of even a small amount of matter contains an enormous number of atoms, counting them atoms in a sample. Both methods make it easier to "count" large individually would be impossible. quantities by using average size or mass. Avogadro's number can also be used as a conversion factor: Think of it like counting in dozens. If you know how many dozens you have, you can quickly figure out how many items by multiplying by 12. Similarly: If you know moles, you can find the number of particles by multiplying by 6.022 x 1023. If you know the number of particles, you can find the moles by dividing by 6.022 x 1023. How Does Mol Relate to the Masses of Elements? When we talk about 1 mole of any element, we mean it contains 6.022 x 10 23 atoms. So, 1 mole of carbon, 1 mole of oxygen, and 1 mole of gold all have the same number of atoms: 6.022 x 1023. Even though 1 mole of each element has the same number of atoms, the mass of each mole varies depending on the element. This is because atoms of different elements have different sizes and weights. For example, 1 mole of carbon atoms weighs about 12 grams, while 1 mole of oxygen atoms weighs about 16 grams. Think of it like having 12 quarters and 12 dimes. Even though you have the same number (12 of each), the quarters weigh more overall because each quarter is heavier than a dime. Similarly, 1 mole of heavy atoms (like lead) will weigh more than 1 mole of lighter atoms (like hydrogen). Molar Mass Molar mass is the mass of 1 mole of an element or compound. Since 1 mole contains 6.022 x 1023 particles, molar mass is the weight of that many atoms or molecules, measured in grams. New Section 1 Page 2 To find the molar mass, look at the atomic mass of an element on the periodic table. For example, carbon has an atomic mass of 12 amu (atomic mass units). To get the molar mass, just change "amu" to "gras per mole (g/mol)." So carbon's molar mass in 12 g/mol. This means that 1 mole of carbon atoms (or 6.022 x 1023 atoms) weighs 12 grams. Atomic masses on the periodic table are based on a relative scale, meaning they show how heavy one atom is compared to another. So, even if we change from amu to g/mol, the relative differences between elements stay the same. Molar masses are often given to 4 significant figures (for example, 40.08 g/mol), which means the measurement is accurate to that many digits. To find the molar mass of a compound, we add up the molar masses of each atom in the formula. The molar masses of individual atoms are found on the periodic table in atomic mass units (amu), but for compounds, we express them in grams per mole (g/mol). For example, take CaF2; 40.08 g/mol + 2 (19.00 g/mol) = 78.08 g/mol Moles allow us to change particles to grams as definitive units of measure (g/mol). Both the mol and molar mass can be use as conversion factors; Hoe many moles of lead does 15.0g of Pb represent? Percent Composition of Compounds Precent composition tell you the percentage by mass, percent meaning "out of 100," of each element in a compound. For example, if you know a compound's percent composition, you know what percent of its total mass is made up of each element. When we calculate percent composition, we treat the compound's molar mass as the "100% total" mass. Each element's contribution to the total mass can then be expressed as a percentage. Sample size doesn't matter as percent composition is the same no matter how much of the compound you have. For instance, water (H 2O) is always about 11% hydrogen and 89% oxygen, whether you have one drop or a gallon. You can find the percent composition it two ways; 1. Using the chemical formula: If you know the formula (e.g., H₂O), you can calculate percent composition directly. 2. Using experimental data: If you have experimental measurements of the element masses, you can use those to calculate the composition. ○ Step 1: Find the molar mass of the compound by adding up the masses of all atoms in its formula. ○ Step 2: For each element, divide its total mass in the compound by the compound’s molar mass, then multiply by 100 to get a percentage. New Section 1 Page 3