# Homework 63

- Page ID
- 29246

Assigned Questions: 10.34,5.75

**Question 10.34:**

Observe the following exchanges of energy. Identify whether each exchange is an example of heat or work. For each system, determine whether ΔE is positive or negative.

**a) **A cake is baked in an oven (The cake is the system)

**b) **Wood is burned for a fire (The wood is the system)

**c) **A boy kicks a chair, it is displaced in the direction of force applied. (The chair is the system)

**Solution**

**a) **The system is an example of heat, and because the cake gains heat, ΔE is positive.

**b) **The system is an example of heat, and because wood gives off heat, ΔE is negative.

**c) **The system is an example of work, and because work is done on the chair, ΔE is positive.

**Question 5.75**

Calculate the number of moles in each of the following examples.

**a) **402.5 mg of NO_{2}

**b) **2.7 kg of H_{2}O

**c) **323 g of CBr_{4}

**d) **2.9 kg of CaO

**Solution**

**a) **

__Explanation:__

** **First, calculate the molar mass of NO_{2.}

*14.007g/mol N + 2 (15.999 g/mol O) = 46.0055 g/mol *

Now, convert the sample from mg to g.

402.5 _{2} | 1 g | 0.4025 g NO_{2} |

1000 |

Finally, use the molar mass to determine how many moles your sample size contains.

.4025 g NO_{2} | 1 mol | 0.0087 moles of NO_{2} |

46.0055 g |

This gives us the final answer, **0.0087 moles of NO**_{2. }

From this, we can deduce that:

Moles of substance = Mass of substance (g) / Molar mass of substance

**b) **

**150 moles of H _{2}O**

**c)**

**0.974 moles of CBr _{4}**

**d)**

**52 moles of CaO**