Chemistry

What is 0.00042 in scientific notation?

Expand 4.2×10^8 to standard notation.

How many significant digits are in 0.00150?

How many significant digits are in 200?

Complete the operation and report your answer with the correct number of significant digits: 2.5 + 3.25

Complete the operation and report your answer with the correct number of significant digits 3.25/ 4

What is 655 nm in meters?

What are isotopes?

Which subatomic particle has no charge?

Alkali metals are the elements found in Group 1 of the periodic table. This group includes lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). They are characterized by having one valence electron, which makes them highly reactive and prone to forming positive ions.

Noble gases are the elements in Group 18 of the periodic table. They are also known as inert gases or rare gases. The noble gases include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn)

What are polyatomic ions?

Atoms form ions by loss or gain of electrons. The ions that contain a greater number of protons than electrons are known to hold a net positive charge. These ions are commonly referred to as cations. On the other hand, the ions that contain a greater number of electrons than protons are known to hold a net negative charge. These ions are commonly known as anions.

If an ion is made up of only one type of atom (each holding some net charge, positive or negative), it can be referred to as an atomic ion or a monatomic ion. On the other hand, if an ion is made up of two or more atoms, it can be referred to as a polyatomic ion or a molecular ion.

Empirical Formulas

An empirical formula tells us the relative ratios of different atoms in a compound. The ratios hold true on the molar level as well. Thus, H₂O is composed of two atoms of hydrogen and 1 atom of oxygen. Likewise, 1.0 mole of H₂O is composed of 2.0 moles of hydrogen and 1.0 mole of oxygen. We can also work backwards from molar ratios since if we know the molar amounts of each element in a compound we can determine the empirical formula.

Example 3.4.1: Mercury Chloride

Mercury forms a compound with chlorine that is 73.9% mercury and 26.1% chlorine by mass. What is the empirical formula?

Solution

Let's say we had a 100 gram sample of this compound. The sample would therefore contain 73.9 grams of mercury and 26.1 grams of chlorine. How many moles of each atom do the individual masses represent?

For Mercury:

(73.9g)×(1mol200.59g)=0.368moles

For Chlorine:

(26.1g)×(1mol35.45g)=0.736mol

What is the molar ratio between the two elements?

0.736molCl0.368molHg=2.0

Thus, we have twice as many moles (i.e. atoms) of Cl as Hg. The empirical formula would thus be (remember to list cation first, anion last):

HgCl2

Chemical Formula from Empirical Formula

The chemical formula for a compound obtained by composition analysis is always the empirical formula. We can obtain the chemical formula from the empirical formula if we know the molecular weight of the compound. The chemical formula will always be some integer multiple of the empirical formula (i.e. integer multiples of the subscripts of the empirical formula). The general flow for this approach is shown in Figure 3.4.1 and demonstrated in Example 3.4.2.

List of polyatomic ions

• Ions made up of more than one atom are known as polyatomic ions.

• Ions with positive charge are called cations.

• Ions with negative charge are called anions.

  phosphite	PO33−
  phosphate	PO43−

  ammonium	NH4+
  nitrite	NO2−
  nitrate	NO3−
  sulfite	SO32−
  sulfate	SO42−