Today was a beautiful day! The seas were calm, the skies were clear, and we started around-the clock analyses. OK, so that last one wasn’t really beautiful, but it does mean we have hita major milestone for the expedition. Today we arrived at our first process station, which is an area of the ocean we want to study more thoroughly. We’ll spend about four days at this location, collecting lots of samples and performing numerous tests and analyses. For me, this means more time with my buddy, the NOx Box, and making a bunch of solutions and standards.
As I look around, all I see beyond the ship is water, the inspiration for today’s topic. Water is one of the most abundant molecules on Earth. The world ocean comprises 71% of the Earth’s surface area and is completely essential for life on our planet (this isn’t the total amount of water on Earth – there’s also ice and other smaller bodies of water).
If you’ve ever tasted seawater (intentionally or accidentally), the first thing you probably noticed is that it was salty. Unlike pure water (H2O only), seawater contains a lot of salt. This salinity, which is the concentration of all the chemicals in the ocean, gives seawater some different properties than freshwater. Two of these are density and freezing point. Density, as my students should remember, is the amount of mass per unit of volume (usually mL, L, or cm3). The density of all water depends on temperature and pressure. At 20°C, the density of water is about 1g/mL (1kg/L). When you decrease the temperature, the density also decreases. Think about a glass of ice water – are the ice cubes at the top or bottom of the glass? From experience, you know that ice floats. This is due to its density being less than that of the liquid it’s in – less dense will always float on top of a denser substance (this is actually a unique property of water; for most substances, the solid form is more dense than the liquid).
Seawater’s density, like that of fresh water, also depends on temperature and pressure but also on salinity. As the salinity of water increases, the density also increases. This partially explains why, at the sea’s surface, the density of water is about 1.027 kg/L. It also helps explain an interesting difference between pure water and seawater in their maximum densities. Pure water is at its densest at 4°C, whereas seawater is at its densest right before freezing.
Another difference between fresh water and seawater that’s caused by salt ions is the freezing point, the temperature at which a substance becomes solid. Water’s freezing point is normally 0°C (32°F), but seawater has a freezing point of -1.9°C (28.58°F). This decrease, called a freezing point depression, is one of the colligative properties (a property that depends on the ratio of solute molecules to solvent molecules) of all solutions – increasing the amount of dissolved solutes (in this case, salt ions) increases the molality of the solution, which changes the freezing point of the solution. Molality is the mass of solute (what is dissolved in a solution) per kilogram of solvent (what is doing the dissolving). In seawater, the solutes are the salt ions whereas the solvent is water.
By mass, approximately 86% of the salt in seawater is NaCl. The other main ionic components are magnesium, calcium, potassium, and sulfate. Scientists have experimentally determined the molality of the each of the major ions in the ocean. From that, we can calculate the molar concentration of each salt component in the ocean.
Let’s do some math!
The molality of sodium ions in the ocean is 10.76 g/kg. What is the molarity (molar concentration) of sodium? Assume the density of seawater is 1.027 kg/L.
To solve this, we need to convert g/kg to mol/L – both of which are simple to do!
Step 1: Write down what you know
Solute = sodium
Solvent = water
Molality = 10.76 g/kg (this means 10.76 g Na+ / 1 kg water)
Density = 1.027 kg/L
Molar mass of sodium (from your Periodic Table!) = 23 g/mol
Step 2: Convert the mass of sodium to moles
10.76 g Na x (1 mol Na / 23 g Na) = 0.4678 mol Na+
Step 3: Convert mass of water to liters (use density!)
D= 1.027 kg/L
Mass = 1 kg (from the molality)
The equation for density is D = mass/volume, if we rearrange that to solve for volume, we get the equation V = mass/D
V = 1 kg / (1.027 kg/L) = 0.973 L
Step 4: Solve for Molarity!
M = mol/L
M = 0.4678 mol / 0.973 L = 0.481 M
This means that in every liter of water, there are 0.481 moles of sodium ions.
Now, I want you to calculate the molar concentration of chlorine: molality = 19.35 g/kg, molar mass of Cl = 35.45 g/mol, density = 1.027 kg/L. Just follow the same steps! (you should get 0.561 M Cl–)
Until next time,