Determination of the Formula of a Hydrate. Ms. Kiely Coral Gables Senior High School IB Chemistry SL

Transcription

1 Determination of the Formula of a Hydrate Ms. Kiely Coral Gables Senior High School IB Chemistry SL

2 Bell-Ringer Read pg 23 and then answer the following questions: 1. What is a hydrated salt, a.k.a hydrated crystal? 2. Find out and write down the formulas of two hydrated crystals. 3. Suggest two ways in which these crystals may change upon heating. Lab Quiz NEXT TUESDAY (because 7th period has junior assembly)

3 Announcement 7th period: Rebuild for HW due NEXT CLASS before Junior Assembly - no exceptions. All classes: There will only be tutoring on Wednesday this week until 3:15.

4 Hydrated Crystals Hydrated crystals are also called hydrated salts because they are ionic compounds. These ionic compounds, however, contain a fixed ratio of water molecules located inside of their crystalline structure. These water molecules are known as the formula unit s water of crystallization.

5 The formula of the hydrated salt is shown with a dot before the number of water molecules. For example, calcium chloride tetrahydrate: CaCl₂ 4H₂O Molecule versus Formula Unit The formulas of ionic compounds are already written in simplest form, therefore their formulas are already empirical formulas in nature. Only molecules have molecular formulas, where atom ratios may not be in simplest form. We solve for the empirical formulas of molecules to determine their molecular formulas. Molecules are the result of bonded nonmetal atoms, whereas ionic compounds contain nonmetal atoms bonded to metal atoms. That is why we refer to individual ionic compounds as formula units, not as molecules.

6 Hydrated Crystals Today we will be heating a hydrated salt and will use the change in mass (assuming the water evaporates) to determine the formula of this hydrated salt. The only thing we know so far is that the formula unit present in this hydrated salt is barium chloride, BaCl₂ Therefore, we are solving for the x in the following hydrated salt formula: BaCl₂ xh₂o -We will calculate the mass of the ionic salt present in our sample, and then calculate the mass of water present in our sample. -We will then convert those mass values to moles and will use the mole ratios to determine the formula of the hydrated salt.

7 Error in Data -Random errors: errors out of your control, caused by limitations of the measuring apparatus and other inevitable uncontrollable variables. For instance, we weighed our samples on a digital scale. If we had done many trials, we would want to measure our samples many times so as to account for differences in the amount of hydrated salt poured into the sample to begin with. All measuring apparatus comes with uncertainty. For a digital apparatus, such as our scale that reads to three decimal places, the uncertainty lies between and

8 Error in Data -Systematic errors: an error introduced into an experiment by the apparatus or procedure, which cannot be reduced by repeating trials. Systematic errors result in a loss of accuracy, i.e. the measured value is further away from the true value. They are errors that result in a problem with the system! For example, imagine you are using a balance that has not been calibrated recently, but you do not know this. All of your readings come out 1.00g too high, but again, you would not know this was incorrect, and performing several trials for an average would still not erase that all readings are 1.00g too high. Systematic errors can only really be identified by comparison with accepted literature values! Therefore something called percentage error is calculated.

9 Percent Error Percentage error = experimental value - accepted value accepted value x 100 We use percentage error when we are trying to determine whether our data potentially contains solely random errors or whether it additionally may contain systematic errors. In order to do so, we must compare percentage error with percentage uncertainty. Percentage uncertainty = smallest reading possible from apparatus experimental value x 100