### Figure 6.1 “Water Molecules” shows that we need 2 hydrogen atoms and also 1 oxygen atom to do 1 water molecule. If we desire to do 2 water molecules, we will need 4 hydrogen atoms and also 2 oxygen atoms. If we desire to make 5 molecules of water, we require 10 hydrogen atoms and 5 oxygen atoms. The proportion of atoms us will must make any number of water molecule is the same: 2 hydrogen atoms to 1 oxygen atom.You are watching: Number of hydrogen atoms in water

Figure 6.1 Water Molecules. The proportion of hydrogen atoms to oxygen atoms supplied to do water molecules is always 2:1, no matter how countless water molecules are being made.

One trouble we have, however, is that it is very difficult, if not impossible, to organize atoms one at a time. As declared in the introduction, we address billions of atom at a time. How deserve to we save track the so many atoms (and molecules) at a time? We carry out it by using mass rather than through counting separation, personal, instance atoms.

A hydrogen atom has a fixed of roughly 1 u. One oxygen atom has actually a massive of approximately 16 u. The proportion of the massive of an oxygen atom come the massive of a hydrogen atom is therefore approximately 16:1.

If we have 2 atoms of each element, the ratio of your masses is about 32:2, which reduces come 16:1—the exact same ratio. If we have 12 atom of every element, the proportion of their total masses is roughly (12 × 16):(12 × 1), or 192:12, which additionally reduces come 16:1. If we have actually 100 atom of every element, the proportion of the masses is approximately 1,600:100, which again reduces come 16:1. As lengthy as we have equal number of hydrogen and also oxygen atoms, the proportion of the masses will always be 16:1.

The very same consistency is seen as soon as ratios of the masses of other facets are compared. For example, the proportion of the masses that silicon atoms to same numbers that hydrogen atoms is constantly approximately 28:1, when the ratio of the masses of calcium atom to equal numbers of lithium atoms is roughly 40:7.

So we have developed that the masses of atom are constant with respect to every other, as long as we have actually the same variety of each kind of atom. Consider a an ext macroscopic example. If a sample has 40 g the Ca, this sample has the same variety of atoms as there space in a sample of 7 g of Li. What us need, then, is a number that represents a convenient amount of atoms so we deserve to relate macroscopic amounts of substances. Clearly even 12 atoms space too couple of because atom themselves space so small. We require a number the represents billions and billions of atoms.

Chemists use the ax mole to stand for a large number of atoms or molecules. Simply as a dozen indicates 12 things, a mole (mol) represents 6.022 × 1023 things. The number 6.022 × 1023, dubbed Avogadro’s number after the 19th-century chemist Amedeo Avogadro, is the number we use in chemistry to stand for macroscopic quantities of atoms and also molecules. Thus, if we have actually 6.022 × 1023 O atoms, we say we have actually 1 mol of O atoms. If we have actually 2 mol the Na atoms, we have actually 2 × (6.022 × 1023) Na atoms, or 1.2044 × 1024 Na atoms. Similarly, if we have 0.5 mol the benzene (C6H6) molecules, we have 0.5 × (6.022 × 1023) C6H6 molecules, or 3.011 × 1023 C6H6 molecules.

### Note

A mole represents a very big number! If 1 mol of quarters were stacked in a column, it might stretch earlier and forth between Earth and also the sunlight 6.8 billion times.

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Notice that we are applying the mole unit to different varieties of chemical entities. In this examples, us cited mole of atoms and moles of molecules. The word mole represents a variety of things—6.022 × 1023 of them—but does no by chin specify what “they” are. They have the right to be atoms, formula devices (of ionic compounds), or molecules. That details still demands to it is in specified.

Because 1 H2 molecule contains 2 H atoms, 1 mol of H2 molecules (6.022 × 1023 molecules) has actually 2 mol of H atoms. Using formulas to indicate how many atoms that each aspect we have in a substance, we deserve to relate the variety of moles of molecules to the number of moles the atoms. Because that example, in 1 mol of ethanol (C2H6O), we have the right to construct the complying with relationships (Table 6.1 “Molecular Relationships”):

Table 6.1 molecule Relationships1 Molecule of C2H6O Has1 Mol the C2H6O HasMolecular Relationships
2 C atoms2 mol the C atoms frac2,mol,C,atoms1,mol,C_2H_6O,molecules or frac1,mol,C_2H_6O,molecules2,mol,C,atoms
6 H atoms6 mol that H atoms frac6,mol,H,atoms1,mol,C_2H_6O,molecules or frac1,mol,C_2H_6O,molecules6,mol,H,atoms
1 O atom1 mol that O atoms frac1,mol,O,atoms1,mol,C_2H_6O,molecules or frac1,mol,C_2H_6O,molecules1,mol,O,atoms

### Example 1

If a sample consists of 2.5 mol the ethanol (C2H6O), how plenty of moles that carbon atoms, hydrogen atoms, and also oxygen atom does the have?

Solution

Using the relationship in Table 6.1 “Molecular Relationships”, we apply the ideal conversion element for each element:

Note exactly how the unit mol C2H6O molecule cancels algebraically. Comparable equations deserve to be constructed for determining the number of H and also O atoms:

2.5, lap————————mol,C_2H_6O,molecules,×,frac6,mol,H,atoms1, lap—————————mol,C_2H_6O,molecules,=,15,mol,H,atoms

2.5, lap————————mol,C_2H_6O,molecules,×,frac1,mol,O,atoms1, lap—————————mol,C_2H_6O,molecules,=2.5,mol,O,atoms

### Example 2

How plenty of formula systems are current in 2.34 mol the NaCl? How countless ions are in 2.34 mol?

Solution

Typically in a problem like this, we begin with what we space given and also apply the proper conversion factor. Here, us are given a quantity of 2.34 mol that NaCl, to which we can use the meaning of a mole together a switch factor:

2.34, lap———-mol,NaCl,×,frac6.022,×,10^23,NaCl,units1, lap————-mol,NaCl,=,1.41,×,10^24,NaCl,units

Because there space two ion per formula unit, there are

1.41×10^24, lap————-NaCl,units,×,frac2,ions lap————-NaCl,units,=,2.82,×,10^24,ions