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Understanding Derived Quantities and Density in Chemistry

In this lesson, we explore derived quantities, which are obtained by combining two or more basic values. The concept of derived units is introduced, illustrating how units are formed this way. Using the ideal gas law (PV = nRT), we solve for temperature (T) as a derived quantity with specific units (Kelvin). Additionally, we delve into density, defined as mass per unit volume, presenting examples to clarify its calculation using the formula d = m/V. Students will learn to convert measurements and apply these concepts through provided homework problems.

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Understanding Derived Quantities and Density in Chemistry

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  1. Chemistry Unit 2 Lesson 3

  2. Derived quantities (page 23) • Derived Quantity: A number made by combining two or more other values. • Derived unit: a unit which is made by combining two or more units.

  3. Example 1: • Gas law: PV = nRT, • P = Pressure (atm). • V = Volume (L). • n = mol • R = Gas constant = Latm/Kmol). • T = Temperature in Kelvin (273.15 K = 0 ºC). • Solve for T • T = PV/nR • T is the derived quantity having the derived units of K (Latm/(Latm/Kmol X mol)).

  4. Density. • Mass (m) = the quantity of matter in an object. • Density = the mass contained in a given volume of substance. • Break down what is being said --- mass/volume; thus, d = m/V • What are the units of density?

  5. Example • A rock has a mass of 0.2 kg and a volume of 400 mL. What is the density of the rock? Hint, density is usually measured in g/mL. • Set up the equation. • d = g/mL. • convert kg to g while dividing by mL. • d = (0.2 kg) * (1 g / 10-3 kg) * (1 / 400 mL) = 0.5 g/mL

  6. Example 2 • The density of Beryllium is 1.85 g/mL. Calculate the mass of a 0.75 L chunk of Be. • Change L to mL  0.75L = 750 mL • M = DV 1.85 g/mL x 750 mL = 1387.5 g (1.39 Kg)

  7. Homework • 29 a, d. (Page 23) • 31 – 35, 39 (Page 26)

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