# Tutor profile: Ben B.

## Questions

### Subject: Inorganic Chemistry

How many atoms are in 46g of calcium (Ca).

To answer this question, it helps to think backwards to see what needs to be done to achieve what is being asked. The question asks for the number of atoms. This can be achieved using the number of moles of the substance and multiplying it by Avogadro's number (6.02*10^23- the number of atoms in a mole). This being said, its easiest to cover the number of grams of our substance into moles, and multiplying by Avogadro's number. First, we divide the amount of calcium by its molar mass. This is true because molar mass is in units of g/mol, and our measurement is in grams. By dividing one by another, the grams cancel out, and we are left with moles. This would be 46g/40g= 1.15 moles of calcium. Now that we know we have 1.15 moles of calcium, and that there are Avogadro's number of atoms in a mole of a substance, we can multiply the two together to get the final answer. This would look like 1.15*(6.02*10^23)= 6.92*10^23 atoms.

### Subject: DAT

A compound's empirical formula is NO2 and the molar mass is 92g. What is the compound's molecular formula?

The empirical formula is the formula with the smallest number of atoms in the correct proportion. If our empirical formula is NO2, our compound could be NO2, or N2O4, or N3O6, and so on, because they all have the same proportion of those atoms: 1 N and 2 O. Because this is the case, and we know the molar mass to be 92g, then we can multiply the mass of our empirical formula by a whole number to the 92g of the final molecule. To obtain the mass of the empirical formula compound, we must add the molar mass of 1 N and 2 ). This would be (1*14) + (2*16) = 14+32 = 46g. We need to find a number that we can multiply this by to get a new formula with the same ratio of N to O, which is 1 : 2. The easiest way is to divide the number we need (92) by the number we got from the empirical formula (46). 92/46 = 2. This is our multiplier. For the last step, we multiply all of the values of our empirical formula by the multiplier. There was 1 N originally, so now its 1*2 = 2N. There were 2 O's originally, so the molecular formula will have 2*2= 4 O. Our final answer is N2O4. This formula has the molar mass 92g, which you can check, and is a multiple of the empirical formula.

### Subject: Biology

When considering DNA replication, what are Okazaki fragments and why do they exist?

Okazaki fragments are short segments of DNA that are created in unconnected spurts that must be linked via DNA ligase. Okazaki Fragments exist due to the fact that DNA must replicate from the 5' end to the 3' end. When DNA divides, its two strands split disconnect from each other to create what is known as a replication fork. One strand, called the leading strand, can have nucleotides added to it continuously at the 3' end as the fork opens up. The other strand, the lagging strand, runs 5' to 3' in the opposite direction of the fork opening. Because this is the case, DNA polymerases synthesize in the opposite direction of the fork, then wait for the fork the fork to open some more, then repeat the process. Each little bit of synthesized nucleotides is referred to as an Okazaki fragment.

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