Question 1

Given the isotopic data for Hypothetical Element X:

Mass (amu): 62.982 (14.26%), 63.978 (31.66%), 64.973 (8.18%), 65.968 (45.90%)

Calculate the average atomic mass of Element X. Which of the following is closest to the correct value?

$$	ext{Average atomic mass} = \sum (	ext{fractional abundance} 	imes 	ext{isotopic mass})$$

Accepted Answer

$$64.83\ 	ext{amu}$$

Question 2

Hypothetical Element Y has two isotopes: 235.04 amu and 238.05 amu. Its average atomic mass is 238.029 amu. What is the percent abundance of the 235.04 amu isotope?

$$	ext{Let } x = 	ext{fractional abundance of 235.04 amu}$$
$$238.029 = (235.04)x + (238.05)(1-x)$$

Accepted Answer

$$0.70\%$$

Question 3

Which scientist is credited with discovering the neutron, a subatomic particle with no charge and a mass similar to the proton?

Accepted Answer

James Chadwick

Question 4

A sample of water is heated from $$25^\circ\mathrm{C} to$$ $$75^\circ\mathrm{C}. If $$the mass of water is $$100\ \mathrm{g}$$ and the specific heat capacity is $$4.18\ \mathrm{J/g\cdot ^\circ C}$$, how much heat is absorbed?

$$q = m c \Delta T$$

Accepted Answer

$$20,900\ \mathrm{J}$$

Question 5

Which of the following best describes an endothermic process?

Accepted Answer

A reaction that absorbs heat from the surroundings

Question 6

Convert $$250.0\ \mathrm{cal} to $$joules. ($$1\ \mathrm{cal} = 4.184\ \mathrm{J})$$

Accepted Answer

$$1,046\ \mathrm{J}$$

Question 7

Which element is a halogen in period 4?

Accepted Answer

Bromine (Br)

Question 8

Which of the following statements about isotopes is correct?

Accepted Answer

Isotopes have the same number of protons but different numbers of neutrons.

Question 9

Which group on the periodic table contains elements that are shiny, good conductors of heat and electricity, and tend to lose electrons to form cations?

Accepted Answer

Metals

Question 10

A neutral atom of magnesium ($$^{24}_{12}\mathrm{Mg}) $$has how many protons, neutrons, and electrons?

Accepted Answer

12 protons, 12 neutrons, 12 electrons

Question 11

Which of the following best describes the periodic law?

Accepted Answer

The properties of elements are periodic functions of their atomic numbers.

Question 12

Which scientist is credited with organizing the periodic table by increasing atomic number, leading to the modern periodic law?

Accepted Answer

Henry Moseley

Question 13

Which of the following ions is formed when a neutral atom of sodium loses one electron?

Accepted Answer

$$\mathrm{Na}^+$$

Question 14

Which of the following is the correct isotopic symbol for an atom with 20 protons, 22 neutrons, and a $$2+$$ charge?

Accepted Answer

$$^{42}_{20}\mathrm{Ca}^{2+}$$

Question 15

Which of the following best describes a cation?

Accepted Answer

An atom that has lost one or more electrons

Question 16

Which of the following is a metalloid in group 13?

Accepted Answer

Boron (B)

Question 17

If a reaction releases $$500\ \mathrm{J} of $$heat to the surroundings, which term best describes this process?

Accepted Answer

Exothermic

Question 18

Convert $$100.0\ ^\circ\mathrm{C} to $$Kelvin.

Accepted Answer

$$373.15\ \mathrm{K}$$

Question 19

Which of the following best describes the difference between kinetic and potential energy?

Accepted Answer

Kinetic energy is energy of motion; potential energy is stored energy.

Question 20

How many atoms are present in one formula unit of $$\mathrm{Mg}_3(\mathrm{PO}_4)_2$$?

Accepted Answer

13

Average Atomic Mass and Isotopic Abundance Calculations – Step-by-Step Guidance

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