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Why do you think DNA is made up of four bases? How many hydrogen atoms are there in a glass of water? How does the structure of the amino acid glycine change with pH? Do you know of any experiments that we use to discover the properties of DNA? Trisomy 21 is a genetic disease whereby an individual has a third copy of chromosome 21? What do you think causes this, and why would this lead to Down Syndrome? How does a potassium channel discriminate against sodium, even though the ion is smaller in diameter? Hi I'm Lauren, a fourth-year biochemist at Oxford University. On the list are just some of the questions I was asked in my Oxford interview in 2013. During tuition, I'll go through the answers to these and many more with you in depth, working to improve your ability to work through problems logically and to perfect your general interview style. For now, I will walk you through the model answer to my favorite interview question. How many hydrogen atoms are there in this glass of water? You should structure your answer in a logical order, saying your thoughts aloud throughout. First, you'll need to estimate the glass dimensions. Choose measurements that are easy to work with as you're not likely to be given a calculator. I've chosen a 10 cm high and a 5 cm base radius. Now let's calculate the volume of water. Remember that the volume is the area of the base times the height. The area of the base is r2. The answer you should get is 750 cm3. Converting this into liters is not 0.75 or 750 milliliters. The density of water is 1 gram per centimeter cubed so we know that the mass of water in this glass is 750 grams. The next thing to do is to calculate the number of moles of water molecules present. Remember the important formula you've learned in chemistry moles equals grams over molecular weight. Water is oxygen and two hydrogen's so its molecular weight is 16+2 or 18. So the number of moles is 750/18. Now that you have the moles, you need to work out the number of molecules. Remember, we can do this using Avogadro's constant. One mole contains 6.021023 molecules of water. You won't be expected to know this number, don't worry. Your interview will be there throughout to prompt you with all this information if you get stuck. The final thing to remember is that each molecule of water contains two hydrogen atoms. So your final answer will be 51025 atoms of hydrogen. If you prefer, you can leave your answer in an unsimplified form but always simplify if you can. And that is how you work out the number of hydrogen atoms in the glass of water. I hope this video was helpful.