Moles and Molar Mass - AP Chemistry

This question requires an understanding of what avogadro's number actually represents. Avogadro's number, 6.022 * 1023 is the number of things in one mole. The question indicates that there is 1 mole of H2. Thus there are 6.022 * 1023 molecules of H2. However the question is asking for the amount of atoms in 1 mole of H2. Thus we must consider the makeup of an H2 molecule, where we see that it is a diatomic molecule. Thus we must multiply 6.022 * 1023 by 2 to calculate the number of individual atoms present in 1 mole of H2. We find our answer to be 1.2044 * 1024.

To find the molecular weight, you need to add together the atomic masses of each of the atoms. 1 sulfur is 32g/mol, 1 oxygen is 16g/mol, and 2 chlorine is 71g/mol (each Cl atom is 35.5g/mol). These values are taken from the periodic table. Added together, they make 119g/mol.

PCl5, BF3, and C6H12O6 are not ionic compounds, and can thus be eliminated because "formula weight" implies that we are dealing with an ionic compound. This leaves only the correct answer, KCl, which can also be verified as correct by calculating the weight. K is 39g, while Cl is 35.5g. Together, this gives a total weight of approximately 74.5g.

4.37times $10^{-3}$ghspace{1 mm}CH_3CH_2OHtimes $\frac{1hspace{1 mm}$molehspace{1 mm}CH_3CH_2OH}{46hspace{1 mm}ghspace{1 mm}CH_3CH_2OH}times$\frac{6.022times $10^{23}$$hspace{1 mm}moleculeshspace{1 mm} CH_3CH_2OH}{1hspace{1 mm}molehspace{1 mm}CH_3CH_$2OH}=5.72times10^{19}$hspace{1 mm}moleculeshspace{1 mm}CH_3CH_2OH

$5.34times10^{15}$hspace{1 mm}particleshspace{1 mm}C_2H_6times $\frac{1hspace{1 mm}$molehspace{1 mm}C_2H_6}{6.022times $10^{23}$hspace{1 mm}particleshspace{1 mm} C_2H_6}times $\frac{30.0hspace{1 mm}$ghspace{1 mm}C_2H_60=}{1hspace{1 mm}molehspace{1 mm}C_2H_6}=2.66times $10^{-7}$ghspace{1 mm}C_2H_6

To determine the number of hydrogen atoms, divide the mass of ethanol by its molar mass to get moles of ethanol.

Multiply this by six atoms of hydrogen per molecule of ethanol and by Avogadro's number to get the number of hydrogen atoms.

Use the density of water, the molar mass of water, and Avogadro's number to calculate the number of molecules of water.

We have 500mL of water. Use the density to convert this to grams; then use the molar mass of water to convert this to moles.

There are two moles of hydrogen atoms per one mole of water.

Finally, multiply by Avogadro's number.

Lithium iodate is comprised of and . The anion refers to the negative ion, so we are looking for the percent by mass of .

The total molar mass of lithium iodate is .

The anion, , contributes 190.9g to the total mass.

Find the percent by mass by dividing the mass of the anion by the total molecular mass.

To solve this problem we use the concept of atomic mass and Avogadro's number:

And the number of Ag atoms will be:

The chemical formula has a 2 right outside of the parentheses surrounding nitrate, so we need to distribute/multiply that 2 to everything inside the parentheses. That means that we have 1 calcium, two nitrogens, and six oxygens, and we need to add up all those masses to get the molar weight.

Avogadro's number can change: atoms, molecules, particles, formula units, and photons into moles (and vice versa). But in order to change grams to moles or moles to grams, the molar mass is needed.

In order to find the molar mass of potassium sulfide, you must first write out the chemical formula for potassium sulfide. The symbol for the potassium ion is , and the symbol for sulfur is . We know that this is an ionic compound because we have a metal involved (potassium) with a nonmetal (sulfur), so we must balance the net charge while making sure both atoms have a full octet. Since potassium is in group I, we know that it can easily lose one electron to fill its outer shell of electrons. Sulfur is in group VI, so we know that it can easily gain two electrons to fill its valence shell of electrons. It would take two potassium atoms to donate one electron each to sulfur to make this compound stable. Thus the chemical formula would be . Use the periodic table to find the molar mass.

To solve this problem we need to do some stoichiometry. Once we have gathered all the conversion information needed it becomes a matter of stringing the equation together and making sure our units cancel out to give us the units desired. Trying to tackle this problem stepwise could soon become confusing as there are more than a few steps involved.

Calculation for the Mass of Uranium Mined to Supply the Energy Needs of 1 American for a Year:

It is worth noting that this is the mass of U and doesn't account for the fact that the U mined is not present as a pure element and therefore in reality the mass of material mined overall to get that 75.71g is much greater.

Calculation for the mass of Thorium Mined to Supply the Energy Needs of 1 American for a Year:

The mass of Thorium mined is much less than the mass of Uranium mined mainly because of the concept of isotopic abundance. The isotope of thorium that is useful as nuclear fuel accounts for practically 100% of the naturally occurring thorium, whereas the useful isotope of uranium accounts for only 0.72% of the naturally occurring uranium. Also it is worth noting that like uranium, naturally occurring thorium is not present as a pure compound so therefore in reality the mass of material mined to get that 0.59g is also much greater. However it is also worth noting that thorium is much more abundant than uranium.

The formula for ammonium hydroxide is . To find molar mass, add up the individual masses of each element, and remember to multiply that mass by the molar ratio of that element.

First, write the formula for oxalic acid: . We know that this formula is correct because when we have an acid with an "-ic" ending and no "hydro-" prefix, that means it's coming from a polyatomic ion with an "-ate" ending. So we know that oxal"ic" acid is derived from the oxal"ate" polyatomic ion. The formula for that ion is . Putting a hydrogen atom in front of this ion, and balancing the charges, we get .Then add the atomic masses of each of the individual elements in their respective ratios, to get the molar mass for the whole compound.

Use dimensional analysis to convert from moles to molecules.

Since the given measurement of has two significant figures, the answer must also have two significant figures.

The reason the answer has 2 significant figures, is because the given measurement has 2 significant figures. Remember: in non-decimal numbers, zeros only count if in between non-zero numbers.

First, find how many moles of chlorine are present in the sample:

Because the ratio of moles of to moles of is 1:1, there are also 2 moles of . The next step is to find the mass of 2 moles of .

Find the number of moles of each element.

If the chemist has 2.0 moles of the compound, 1 mole would contain half as much. So 1mol of carbon, 4mol of hydrogen, and 2mol of oxygen, which forms acetic acid