1. Which of the following are physical changes and which are chemical changes: (a) boiling soup for lunch, (b) burning wood in a fireplace, (c) grinding up sugar crystals into a powder, and (d) water evaporating from a swimming pool?

All of the above processes involve physical changes except (b), the burning of wood, which is the chemical combination of the fuel with oxygen from the air.

2. If a given chemical reaction takes 6.0 seconds to complete when the temperature is 35°, what can be said about how long would the same reaction take if the temperature dropped to 15°?

The length of time for the reaction to take place will be greater than 6.0 seconds, because a decrease in temperature will decrease the rate of the reaction and hence increase the time it takes.

3. Write and balance the equation for the complete combustion of the hydrocarbon octane (C₈H₁₈), where carbon dioxide (CO₂) and water (H₂O) are formed.

This reaction requires the combination of oxygen with octane, so the equation would be:

C₈H₁₈ + O₂ --> CO₂ + H₂O

We can see that there are 8 carbon atoms on the left side of the equation, so our first attempt will be to place 8 carbons on the right side by placing an 8 as a coefficient in front of the carbon dioxide. We can also balance the hydrogen atoms by placing a 9 in front of the water.

C₈H₁₈ + O₂ --> 8 CO₂ + 9 H₂O

This balances everything but the oxygen on the left side, and since there are 25 oxygen atoms on the right we would have to multiply the two atoms of oxygen on the left by 25/2, but we are not allowed to use fractional coefficients so we must multiply all coefficients by 2 in the denominator of the fraction, including the "1" in front of the octane. This gives us a complete equation of:

2 C₈H₁₈ + 25 O₂ --> 16 CO₂ + 18 H₂O

4. How many moles of sulfuric acid would be present in a 2.50 kg sample of this material?

Here we know the mass of the sample to be 2.50 kg or 2500 g, and from part (b) of the above problem we know the molar mass for this compound to be 98.1 g/mol. To find the number of moles we must divide the total mass of the sample by the formula mass.

# moles = 2500 g / 98.1 g/mole = 25.5 moles

5. How many sulfuric acid molecules are present in this 25.5-mole sample?

Since we already know the number of moles present, we need only multiply by the number of entities in any one mole sample to get the number of molecules in the acid sample.

# molecules = # moles (6.02 x 10²³ molecules/mole)

= 25.5 (6.02 x 10²³) molecules = 1.54 x 10²⁵ molecules

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