General USMDO

How is a kilocalorie (kcal) defined?

The energy required to raise the temperature of 1 liter of water by 1 degree C

How many kJ are in 1 kcal?

About 4.2 kJ (1 kcal = 4.2 kJ)

Which four elements make up about 96% of a cell's mass?

Carbon (C), hydrogen (H), nitrogen (N), and oxygen (O)

What is an atom?

The smallest unit of a chemical element that retains that element's distinctive chemical properties

What does an atom's nucleus contain and what surrounds it?

Positively charged protons and neutral neutrons; a cloud of negatively charged electrons surrounds it

What determines an atom's chemical properties?

The number and arrangement of its electrons

When is an atom most stable?

When its outer electron shell is completely filled

What is a covalent bond?

A bond formed when a pair of outer-shell electrons is shared between two adjacent atoms

What is a double bond?

A covalent bond in which two pairs of electrons are shared

What is a molecule?

A cluster of two or more atoms held together by covalent bonds

What is an ionic bond?

A noncovalent bond formed when an electron jumps from one atom to another, creating oppositely charged ions held together by mutual attraction

What is electronegativity?

A measure of an atom's attraction for electrons in a bond; unequal sharing makes a bond polar

What are polar molecules and how do they behave in water?

Molecules with uneven charge distribution; they are hydrophilic and dissolve in water

What does hydrophobic mean?

Water-fearing; nonpolar molecules that do not dissolve in water and cluster together

What percentage of a cell is water by weight?

About 70%, so the chemistry of life takes place in an aqueous environment

Why is water a liquid at room temperature while H2S is a gas?

Oxygen is more electronegative than sulfur, so water is highly polar and forms many hydrogen bonds between molecules; H2S does not

What is a hydrogen bond?

A weak electrostatic attraction between a partially positive hydrogen (bonded to an electronegative atom) and another electronegative atom

What are the four main categories of small organic molecules in cells?

Sugars, fatty acids, amino acids, and nucleotides

What is the difference between organic and inorganic molecules?

Organic molecules are carbon-based; inorganic molecules generally are not

What is an acid and what is a base?

An acid donates protons (H+); a base accepts protons

What does the pH scale measure?

Hydrogen ion concentration: pH = -log[H+]; pH 7 is neutral, lower is acidic, higher is basic

What is a buffer?

A solution component that resists changes in pH

What is a hydronium ion?

H3O+, formed when a proton (H+) binds to water

What is Avogadro's number?

About 6 x 10^23, the number of molecules in one mole

What is the molecular weight of a molecule?

The sum of the atomic weights of all its atoms

Do all atoms of the same element have the same number of neutrons?

No. The number of neutrons can vary (isotopes), and it influences whether a nucleus is stable or radioactive

What is a van der Waals attraction?

A weak, short-range attraction between molecules arising from fleeting charge fluctuations

What is an electrostatic attraction?

An attraction between opposite charges, e.g. between oppositely charged ions or groups

Which of the following is NOT one of the four elements that make up about 96% of a cell's mass? A. Carbon B. Hydrogen C. Nitrogen D. Phosphorus

D. The four are C, H, N, and O

Which statement about atoms is NOT correct? A. The nucleus contains protons and neutrons B. A neutral atom has equal numbers of protons and electrons C. All atoms of the same element have the same number of neutrons D. Electron arrangement determines chemical properties

C. Atoms of the same element can differ in neutrons (isotopes)

Which statement about covalent and ionic bonds is NOT correct? A. A covalent bond shares an electron pair B. A double bond shares two electron pairs C. An ionic bond results from an electron jumping between atoms D. A covalent bond forms when an electron jumps from one atom to another

D. That describes an ionic bond

Which of the following is NOT true of water? A. It is polar B. It forms hydrogen bonds C. Cells are about 70% water D. It is a good solvent for nonpolar molecules

D. Polar molecules and ions dissolve in water; nonpolar molecules do not

Which of the following is NOT true of pH and buffers? A. pH 7 is neutral B. Acids donate protons C. Buffers resist changes in pH D. A lower pH means a lower hydrogen ion concentration

D. Lower pH means a higher hydrogen ion concentration

Which statement about energy units is NOT correct? A. 1 kcal raises 1 L of water by 1 degree C B. 1 kcal is about 4.2 kJ C. 1 kJ is about 4.2 kcal

C. 1 kcal is about 4.2 kJ, so 1 kJ is about 0.24 kcal

Which of the following is NOT a weak noncovalent interaction? A. Hydrogen bond B. Van der Waals attraction C. Electrostatic attraction D. Covalent bond

D. Covalent bonds are strong