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.

To solve this question, we need to first convert grams to moles, then moles to atoms. The molecular weight (MW) of oxygen gas,  is

The amount of moles in 1g of oxygen gas is

There are  atoms in  (this is called the Avogadro’s number); therefore, the number of atoms in  of  is

The empirical formula for carbohydrates is , where  is the number of carbon atoms. The question states that molecule A has 6 carbons; therefore,  and the molecular formula for molecule A is . The molecular weight (MW) of this compound is calculated using the molecular weight of each atom. The MW of each atom is obtained from the periodic table and is multiplied by the number atoms.

So, the molecular weight of molecule A is . Since it is also a carbohydrate, molecule B will have the same empirical formula as molecule A. Molecule B has half the molecular weight of molecule A; therefore, molecule B must have half the atoms as molecule A. The molecular formula of molecule B is . Note that MW of  is .

The MW contribution of oxygen to each compound is calculated as follows.

%MW of oxygen for molecule A = 

%MW of oxygen for molecule B =  

Therefore, oxygen contributes to 53% of MW in both compounds.

Recognize that this is a double replacement reaction in which ion pairs will switch. After reaction, copper (II) will now be paired with sulfate, and sodium with flouride according to the (unbalanced) equation below. Remember that sulfate has a charge of -2, and flouride of -1.

Silver (I) nitrate is AgNO₃. Recognizing this allows us to eliminate two answer choices, which incorrectly substitute nitrogen (N) for nitrate (NO₃) or balance the molecular charges incorrectly. Of the two remaining choices, only one is balanced correctly.  

Since the reaction takes place at STP, we can convert liters of hydrogen to moles using a constant.

Using this value, we can find the moles of carbon monoxide produced and convert to molecules by using Avogadro's number.