### Notes:

The Molarity Ranking Tasks Concept Builder is an adjustable-size file that displays nicely on smart phones, on tablets such as the iPad, on Chromebooks, and on laptops and desktops. The size of the Concept Builder can be scaled to fit the device that it is displayed on. The compatibility with smart phones, iPads, other tablets, and Chromebooks make it a perfect tool for use in a 1:1 classroom.

### Teaching Ideas and Suggestions:

We're going to be honest: we do Physics. That's why this is called The Physics Classroom website. And when we do the Teacher's Notes section for our Concept Builders, we typically have a lot to say ... and a lot of resources to point you to. We're not claiming to be ignorant of chemistry; we just don't have a lot of resources here at The Physics Classroom to point you to. And so this page is going to be a lot shorter than our usual page that accompanies our Physics Concept Builders. That's our honest confession.

Solution concentration is a critical concept in a Chemistry course. And the most common concentration expression is molarity. The concept of molarity is used in a Solutions unit, an Equilibrium unit, an Acid-Base unit, and even other units of Chemistry. While molarity is a mathematical quantity, this Concept Builder seeks to approach it at a more conceptual level. While students can certainly pull out their calculators and start calculating molarities, the activity is designed such that the calculator would never have to be used. Simple reasoning about the # of moles of solute, the liters of solution, and the ratio of moles to liters should be sufficient for answering all questions.

There are three difficulty levels that can be engaged in through the Concept Builder. Those three levels are differentiated as follows:

• Apprentice Difficulty Level: Question Groups 1-4 ... Each question provides the number of moles of solute and the volume of three different solutions. Students must rank the solutions according to their molarity.
• Master Difficulty Level: Question Groups 5-8 ... Each question provides the mass of three different solutes, the identity of the solute, and the volume of the solution. Students must rank the solutions according to their molarity.
• Wizard Difficulty Level: Question Groups 9-12 ... Each question provides the amount (grams or number of moles) of three different solutes, the identity of the solute, and the volume of the solution. Students must rank the solutions according to their molarity.

In the early difficulty levels, the given information is straight-forward with a single variable - for instance, the number of moles or the volume (but not both) being used to differentiate between the three given solutions. In the later difficulty levels, there are multiple variables differentiating the three solutions. In all questions, information is presented in the form of a diagram that identifies the solute formula, the amount of solute, and the volume of solution. When a conversion must be made between the grams of solute and the moles of solute, the given amount of grams is such that the conversion can be done by most (thinking) students without a calculator.

The questions from each group are shown on a separate page. Teachers are encouraged to view the questions in order to judge which difficulty levels are most appropriate for their classes.

Like all our Concept Builders, this Concept Builder utilizes a variety of strategies to make each student's experience different. The ordering of questions is random. The Question number assigned to each question is scrambled. For instance, two side-by-side students will not have the same question for question number three. And questions are organized into "groups" with questions within the same group being very similar (for instance, they have the same type of information as "givens") but not identical. And finally, the answer options are always scrambled.

In order to complete a level, a student must correctly analyze each question of that level. If a student's analysis is incorrect, then the student will have to correctly analyze the same or very similar question twice in order to successfully complete the level. This approach provides the student extra practice on questions for which they exhibited difficulty. As a student progresses through a level, a system of stars and other indicators are used to indicate progress on the activity. A star is an indicator of correctly analyzing the question. Once a star is earned, that question is removed from the que of questions to be analyzed. Each situation is color-coded with either a yellow or a red box. A red box indicates that the student has incorrectly analyzed the question and will have to correctly analyze it twice before earning a star. A yellow box is an indicator that the question must be correctly analyzed one time in order to earn a star. Once every question of an activity has been analyzed, the student earns a Trophy which is displayed on the Main Menu. This system of stars and trophies allows a teacher to easily check-off student progress or offer credit for completing assigned activities.

The most valuable (and most overlooked) aspect of this Concept Builder is the Help Me! feature. Each question group is accompanied by a Help page that discusses the specifics of the question. This Help feature transforms the activity from a question-answering activity into a concept-building activity. The student who takes the time to use the Help pages can be transformed from a guesser to a learner and from an unsure student to a confident student. The "meat and potatoes" of the Help pages are in the sections titled "How to Think About This Situation:" Students need to be encouraged by teachers to use the Help Me! button and to read this section of the page. A student that takes time to reflect upon how they are answering the question and how an expert would think about the situation can transform their naivete into expertise.