Given that the molar mass of oxygen is about 16g, and molar mass of manganese is about 55g,  contains 165g of manganese and 64g of oxygen, for a total of 229g.

165g/229g = 0.72

So, the ratio of manganese to oxygen in this compound is 72% manganese by mass.

In order to find the mass percentage of an atom in a molecule, start by finding the total mass of one mole of the molecule. Glucose has 180 grams per mol.

Next, we determine the mass of the carbon atoms in one mole of the molecule. One carbon mole has a mass of 12 grams. Multiplied by the six carbon atoms in glucose gives a mass of 72 grams.

Finally, we divide the mass of carbon by the mass of the molecule.

So, 40% of glucose's mass is made up of carbon.

Aluminum oxide has the formula .

Aluminum has a molecular weight of , and oxygen has a weight of . Using these values, we can calculate the molecular weight of aluminum oxide.

The mass percentage is given by the mass of aluminum divided by the total molecular weight.

Sodium sulfate is given by the formula:

To find the percentage by weight, we will need to divide the mass of sodium in the molecule by the total molecular mass.

Convert the ratio to a percentage.

Start by calculating the percentages of hydrogen in each compound by using molar mass ratios.

Next, take the percentages of the given sample masses to determine the total mass of hydrogen in each.

We see that there is a larger mass of hydrogen in the hydrogen cyanide sample.  

First convert grams of ammonium sulfate to moles, then use the mole-to-mole ratio between nitrogen and ammonium sulfate. Finally, convert moles of nitrogen back into grams.

To find the molecular weight (mass) of a molecule, simply add up the atomic weights of each atom within the molecule. The units used will be amu (atomic mass units) for molecular weight and g/mol for molar mass.

First find the molar mass of water (H₂O). You should be comfortable with the molar masses of hydrigen and oxygen from memory to reduce time on the MCAT exam.

Next, solve for moles.

To calculate the mass percent, calculate the the individual masses of each element. Then divide the mass of sulfur by the total mass of the molecule. Since there is only one sulfur atom in sulfuric acid, the mass of sulfur in one atom is .

The empirical formula for hydrocarbon is , where  is the number of carbons. This is only true if the hydrocarbon has no  bonds or ring structures. The question states that there is one  bond; therefore, the hydrocarbon will lose two hydrogen atoms (). Note that for every  bond and every ring the hydrocarbon is associated with the loss of two hydrogen atoms. If we calculate the molecular weight of hydrocarbons with different  values () we will find that the nine-carbon hydrocarbon (with  bond), nonene, has a molecular weight of . The molecular formula for nonene is ; therefore, the ratio of carbons to hydrogens is 9:18 or 1:2.

You can also calculate the ratio by simply looking at the empirical formula of this molecule (). There will be twice as many hydrogen atoms as carbon atoms; therefore, ratio of carbon to hydrogen will be 1:2.