balance this equation zn hcl zncl h 2 2

HESI A2

Chemistry Hesi A2

1. Balance this equation: Zn + HCl → ZnCl + H2.

A. Zn + 2HCl → ZnCl + H2
B. Zn + HCl → 2ZnCl + H2
C. 2Zn + 2HCl → 2ZnCl + H2
D. Zn + 4HCl → ZnCl + H2

Correct answer: C

Rationale: The given unbalanced equation is Zn + HCl → ZnCl + H2. To balance it, we need to have equal atoms on both sides of the equation. The balanced equation is 2Zn + 2HCl → 2ZnCl + H2. This balanced equation shows that two atoms of Zn combine with two molecules of HCl to form two molecules of ZnCl and one molecule of H2. Choice A is incorrect because it does not balance the equation. Choice B is incorrect as it does not have the same number of atoms on both sides. Choice D is incorrect because it does not balance the equation properly, resulting in an unequal number of atoms on both sides.

2. A radioactive isotope has a half-life of 20 years. How many grams of a 6-gram sample will remain after 40 years?

A. 8
B. 6
C. 3
D. 1.5

Correct answer: C

Rationale: The half-life of a radioactive isotope is the time it takes for half of the original sample to decay. After each half-life period, half of the initial sample remains. In this case, after the first 20 years, half of the 6-gram sample (3 grams) will remain. After another 20 years (total of 40 years), half of the remaining 3 grams will remain, which is 1.5 grams. Therefore, 3 grams will be left after 40 years. Choice A is incorrect as it doesn't consider the concept of half-life and incorrectly suggests an increase in the sample. Choice B is incorrect as it assumes no decay over time. Choice D is incorrect as it miscalculates the remaining amount after two half-life periods.

3. Which of these elements has the greatest atomic mass?

A. Au
B. Ba
C. I
D. W

Correct answer: D

Rationale: Among the elements listed, Tungsten (W) has the greatest atomic mass. The atomic mass of Tungsten is approximately 183.84 atomic mass units (amu), while the atomic masses of the other elements listed are as follows: Gold (Au) is around 196.97 amu, Barium (Ba) is approximately 137.33 amu, and Iodine (I) is about 126.90 amu. Therefore, Tungsten (W) has the greatest atomic mass out of the given elements. Gold (Au) has a higher atomic mass than Barium (Ba) and Iodine (I), making choices A, B, and C incorrect.

4. A salt solution has a molarity of 5 M. How many moles of this salt are present in 0 L of this solution?

A. 0
B. 1.5
C. 2
D. 3

Correct answer: A

Rationale: Molarity is defined as the number of moles of solute per liter of solution. A molarity of 5 M indicates there are 5 moles of salt in 1 liter of the solution. Since the volume of the solution is 0 liters, multiplying the molarity by 0 liters results in 0 moles of salt (5 moles/L x 0 L = 0 moles). Therefore, the correct answer is 0. Option B, 1.5, is incorrect because it doesn't consider the volume being 0 liters. Options C and D, 2 and 3 respectively, are also incorrect as they do not account for the zero volume of the solution. Hence, there are no moles of salt present in 0 liters of the solution.

5. What type of chemical reaction involves the combination of two elements to form a product?

A. Decomposition
B. Combustion
C. Synthesis
D. Double replacement

Correct answer: C

Rationale: A synthesis reaction involves the combination of two or more substances to form a single, more complex product. In the context of chemical reactions, it specifically refers to the combination of two elements to form a compound. Therefore, the correct answer is C. Decomposition reactions involve the breakdown of a single compound into simpler substances (opposite of synthesis). Combustion reactions involve a substance reacting with oxygen to produce heat and light, not the combination of elements. Double replacement reactions involve the exchange of ions between two compounds, leading to the formation of two new compounds, not the combination of two elements.