1. Using the periodic table, determine the result of the following calculation: the number of electrons in the outer shell of a hydrogen atom minus the number of electrons in the outer shell of a helium atom plus the number of electrons in the outer shell of a hydrogen atom.
outer-shell electrons in H: 1 (s shell half filled, only one electron)
outer-shell electrons in He: 2 (s shell is filled – is this the outer shell?)
so 1 – 2 + 1 = 0
This question is a little confusing. Aside from not understanding the context (what is the reason for such a calculation?), I’m not 100% sure what we’re expected to be take as the “outer” shell in He. One might consider the p shell to be the “outer” shell, and this is empty in it’s ground state. But above I assumed the s shell is what was meant.
2. Are the following substances acid, base, or neutral:
sodium bicarbonate (pH = 8.3):base
Blood (pH = 7.4):slightlybasic
milk (pH =6.7):slightyacidic
orange juice (pH = 3.6):acid
milk of magnesia (pH = 10.5):base
Some texts consider milk and blood to be neutral, but technically they’re slightly acidic and basic, respectively.
3. Why does ice float when placed in water?
What roles do hydrogen bonding plan in this phenomena?
Water is less dense in its frozen, solid form than in its liquid state. This is unusual, as most substances are denser when frozen. The reason water ice is less dense than liquid water is that water is a polar molecule held together by the hydrogen bonds between the single O and two H atoms. This bonding results in O having an effective positive charge, and the two Hs effective negative charges. Opposite charges attract, so the water molecules are attracted to each other. When the water freezes, however, it solidifies into a crystal structure that keeps the like-charged O atoms well separated, hence slightly less densely packed than when a liquid.
4. What information does the octane rating of a fuel provide?
When is a fuel with a high octane rating necessary?
Octane, C8H18, is one of the light hydrocarbons, and is present in gasoline. The octane rating of gasoline is a measure of the quantity of a particular octane isomer, iso-octane, in the gas. Iso-octane helps prevent the fuel from spontaneously igniting under pressure, allowing it to be compressed and ignited in a controlled fashion. Engine “knocking” occurs when the fuel spontaneously ignites before being sparked. This is why high-octane fuel is useful in engines that are knocking, or for high performance gasoline engines.